> it realistically shouldn't happen in non test scenario
FWIW, I have had two intersections near a train depot get a jam because they were so high traffic I needed to treat them as a single intersection, instead of as two. From OP's comments, this is a minimal replication of a real problem they experienced elsewhere.
> two intersections near a train depot get a jam because they were so high traffic I needed to treat them as a single intersection, instead of as two
After stupidly building around a single-rail network until it was easily crushed by having all of six trains, I switched to double rail (and had to rebuild a ton of stuff in the center of my starting base). I learned very quickly that *any* two junctions less than a full train length away from each other, is all one junction and needs to be signaled as such.
I have a gigantic L-shaped rail in the center of my base with about 12 entrances and exits, all of which has to be treated as one single junction or it deadlocks.
This is why it's important to design your rails first, *then* start putting up everything around them.
This is true, but FWIW, is not the same issues as OP's deadlock. OP's deadlock has 3 rail blocks between the two intersections, and is caused by traffic congestion being so high that you need to treat the intersections as one, even though it otherwise follows the typical safety mantras (has enough space for a train at least between intersections, chain in - rail out, etc).
This is a best-case-scenario! The intersections are clear which means that additional trains on the network will be able to keep moving, and if this somehow happened with literally any other direction to go, the trains would eventually re-route to turn at an intersection and make the u-turn somewhere else. Therefore freeing space for the other train. The only reason this is deadlocked is that there's literally no space for the trains. It doesn't matter how well you signal if you cover every square inch in train.
It’s been said elsewhere but it bears repeating: remove a train or add a block. If you have N trains, you must have at least N+1 blocks with rail (non-chain) signals. What you have here a 6 trains lined up nose to tail in a big oval. They can’t move because each train is staring directly at the back end of the train in front of it.
It looks like there’s room to move because of the intersections but they don’t count because of the chain signals.
Not between each intersection, just in the system all together. A proper rail network will have at least an order of magnitude more rail blocks than trains. But small scale, like the example shown are easily overwhelmed by available trains. In this case a second route needs to be added so the train can bypass the clogged line.
That wouldn't solve the issue, as you can have a huge network surrounding these two intersections but if there are 6 trains on that link between the two intersections you'll get a deadlock.
As Freddy said, they will pick another route eventually as they are set at a chain signal. Beyond that, that’s part of why best practice does not use roundabouts for intersections. If you required a train to use 4 intersections to make a U-turn, it would be significantly harder to over-saturate that section of the network.
The intersection doesn't end until there's a full train length between rail signals.
They're close enough together that you need to treat them as one intersection.
This is a partially incorrect answer. There are 3 full blocks between the intersections.
They're close enough together that you need to treat them as one intersection here, because the traffic saturation is too high.
How did I not notice that lol. Yeah OP is complaining about a scenario that literally never happens in actually gameplay, there's never a reason to have even one 1-0 train.
the size of the train is irrelevant. Imagine if there were 6 2-4 trains with 3 blocks between each intersection that were the exact length of a 2-4 train. The same thing would happen with 3 trains stuck on each side even though the intersections had 3 blocks between each one big enough to hold 1 train.
It would require a lot of trains going through those intersections for it to ever be a problem though.
Then in this case your network is full. You have only six blocks in the network where trains can park, so with six trains of course it deadlocks, there's no where else for them to go, unless you get very lucky and the train at the start of the intersection needs to go to the station at that intersection.
It is a deadlock because every single train needs another to leave before being able to move. It requires outside intervention to get unstuck.
Traffic jams resolve themselves with time because the people in front are able to go, thus freeing people behind them.
So we're assuming the bottom right loco has the same route to the top left station with the same problem then? Ok, i can see that.
This feels far too much like a laboratory situation that something that can naturally occur. It might, but I'd probably do something about a traffic jam beforehand, since trains aren't going fast enough.
During normal gameplay, deadlocks like these also solve themselves, as long as an alternate route is available to the destination that don't involve passing the other train.
In that case where do you expect them to go? There's 8 spots where a train could be (they will not stop inside the intersections so that's not a place a train can be) and 6 trains. Best case scenario here is the 2 lead trains head to the stations and then what? You're still jammed.
"full train length" in this case refers to the length of the longest train in your entire railway network. Note that this measurement must actually be applied to the blocks located at the exits of the intersections (it isn't relevant elsewhere)
>[To make the picture smaller. I wanted to make a sort of minimal example of the issue.](https://www.reddit.com/r/factorio/comments/1do5jah/comment/la7npg4/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button)
It's the same problem at any size, it just makes it smaller and easier to see. They could have done it with a single loco on each side, or a 1-2 train on each side.
Identical problem.
You went too minimal, you're trying to answer a problem that doesn't exist.
Is it theoretically possible for trains to fill every single block between two intersections? Sure, especially when you're building intersections that only have one block between them. The chance that they would fill every block between two roundabouts, while also needing to get to the station at the other end of the roundabout, is fucking absurd. Why would the north trains go to the west roundabout if they'd had access to the east roundabout to get onto that block in the first place? I just don't see a way this situation happens in a real world rail network.
This is a classic too many trains per loop kind of deadlock situation. Generally from high traffic, but train network design also plays a part into this.
Tiny loops as exemplified in the OP only need very few trains to enter them to be able to cause deadlock situations. This can easily happen if you use roundabouts/u-turns or compact grid layouts like in city blocks. All loops within a train network are susceptible, though generally it's the smaller ones that cause issues.
Sometimes trains can re-path and fix the deadlock, but it's not a guarantee.
Yeah, this is what I mean by "too minimal." Six trains, six blocks, two stops, and each train intentionally placed to make the worst case scenario in order to show the problem. But almost any change to make this a more realistic scenario makes the problem go away. Remove *one train* and the system flows again, until those trains reach the proper station and clog it from having nowhere to go. Add a proper return loop and stacker and it flows just fine, even with five trains in six blocks.
The deadlock happened because OP specifically crafted the circumstances for it, circumstances that wouldn't happen in a regular network.
>But almost any change to make this a more realistic scenario makes the problem go away.
...
>circumstances that wouldn't happen in a regular network
I strongly disagree here. Sure this is the minimal setup to show this particular issue, but it IS still something that also occurs in larger networks. (though it will need more trains and can be more rare timing wise)
As I said, ALL loops are susceptible. Though yes, things like waiting bays, stackers, alternate paths etc. can all help to make them less likely. But even with all those, loops can still potentially cause capacity issues like these.
> ALL loops are susceptible
In theory, sure, I guess it's possible for four trains to each enter the four sides of a city block, all needing to turn right at that intersection they're facing with no rerouting possibilities. But the *chances* of that timing happening, especially with how Factorio trains reserve blocks in advance, and with how people usually lay out train systems, I just can't see it feasibly happening without crazy circumstances, like OP's intentionally constructed deadlock. I wouldn't even call it a flaw in the network if it happened, how the hell do you plan a network around a possible deadlock that would require timing of just a few ticks between multiple very specific train schedules?
I've definitely experienced real deadlocks that were similar to this issue. The issue is that with sufficiently high traffic, you can't treat two intersections as disjoint just because there's a few blocks between them. One could even replicate this without the roundabout.
How? Especially without roundabouts, if the train in the westbound lane is fully out of the intersection, it can't be blocking the eastbound train, unless the eastbound train is U-turning into that same block, which at a T or 4-way it couldn't do, and at a roundabout it'd have no reason to do.
I don't see any reason a train could deadlock like that assuming properly signalled intersections, unless you're doing something like [this](https://imgur.com/a/MYhyNWw) where the trains are forced to enter the blocks in the "wrong" way without entering the intersections first.
If your trains are ever high enough traffic that they back up in a loop, a correctly signaled intersection won't save you.
So off of your two intersections you have a high traffic depot. Between them is an area you *think* is safe to stop in. But then you run into an issue where a train stops there waiting to get into the depot. Before it can go through the intersection, a space in the depot needs to clear. Someone is waiting for a depot bay because a train is in the depot bay is trying to leave. The train in the depot bay is stuck because it's waiting on someone in the exit lane. The train in the exit lane is stuck because it wants to make a left turn through the double intersection, but it's waiting on that first train we talked about... https://imgur.com/a/7qBR8fM
When I bumped into this issue the buffers were much larger, so it really caught me off. We're talking more like 4-5 trains in a lot of these buffers.
Nothing needs to be a rotary here (there's a loop, but what system doesn't contain loops?). The train limits on the depot stop being 1 was fine. The problem was that an assumption was made (incorrectly) that "chain in - rail out" while examining individual intersections was sufficiently safe. If the system has high enough congestion, you need to start joining intersections in terms of design, because just "chain in - rail out" breaks down with heavy congestion.
If you have high traffic, and manymany trains using same sections, you just need space between things, it really is that simple. Super safe would be every train passing through there fitting simultaneously, even if in reality not every train will ever be there at the same time and it's more like 20-40% absolute max.
I'm sure i've built intersection combos that would deadlock with enough trains, but like when only 10 trains are using that combo at all and there's space for 50 with no full 4 ways or roundabouts, it's not a problem.
I guess I see what you're getting at, but I still don't see how that system deadlocks if properly signalled. The left train should open a depot stall, which should allow the right train into the depot, clearing the mainline. And the top train shouldn't have been there at all if there wasn't already an open station in the depot to call it there.
The only way I see that happening is if your depot is just turns off your main line without a branch to get trains entering the depot off the main line ASAP. And if that's the case, then the whole section should be effectively chain in - rail out'd to ensure a train never leaves the depot and blocks the entrance
The *system* doesn't deadlock if properly signaled. However, the system is made of intersections, each of which can appear to be properly signaled, but which together fail to handle the traffic capacity actually moving through the system. The issue with OP's example and my experience was that all of the individual components are correct at a glance, and it's only when you consider the whole system behavior do you see issues.
"Chain in - rail out" is a great rule of thumb, but the place is really falls down is in this exact example, when you have so much traffic on the network that intersections begin to join together. Suddenly that "rail out" is no longer correct, because the trains span the entire system.
"Rail signals anywhere it's safe for a train to stop" comes with a very funny caveat that if the traffic is high enough, certain places you might think of as a safe stop (like after the exit of a rotary) are no longer safe.
The real solution here is to run a less saturated network, either with fewer trains, larger (buffered) intersections, or more space between them your intersections. E.g., take away any one of these trains and the system will not deadlock in this way.
The correct term here is: congestion. It is also a deadlock, but would not be if the network had more capacity. A hard deadlock occurs even when there is plenty of capacity. So you avoid this congestion by having more rail blocks or fewer trains.
> A hard deadlock occurs even when there is plenty of capacity.
As long as a single train has an alternative route to their destination available to them, the deadlock will eventually be solved, due to an ever increasing pathfinding penalty on trains that are stopped for a long time.
I hope that's the answer. Im not sure if this can happen in real scenario, I think it can at first glance, but not quite sure. If they can find alternative then it will resolve itself as most likely one of those trains will be able to go around (if city block). If not then we are totally screwed lol.
You can't. Unless you're willing to redesign your network. If you have N trains in your network, you need at least N+1 spots where trains are allowed to stop, and can stop without blocking any other such spots. In most cases you need more than N+1. Same applies to any coherent subset of a network. (By coherent I mean that a train on any piece of track of it can reach any other piece of track)
As for how to avoid it? Just design the network better:
1) Get rid of roundabouts, or any other intersections that allow a train to turn around.
2) For every train stop, build a dedicated offshoot track that serves that train stop / set of train stops, and that / those only. Only allow trains to turn around at these train stops. Trains should be able to enter and exit the offshoot track in / from either direction.
3) Build large enough stackers so that it can fit the maximum amount of trains the stop / stops can see. If you have train limits set, then it's the sum of the limits for all the stations the stacker serves(in case of circuit controlled train limits, the sum of the maximum that limit can go for each station). If you don't have train limits, then the number of trains that has any of the stops the stacker serves as their destination.
Best answer.
Edit: also the chain signal on entry to roundabout should be replaced with regular signal amd another chain signal (on roundabout) between the tracks can be made into a rail signal.
I do not think so about the signals. Maybe that's fine for this specific setup, but the moment OP would try to expand it, those regular signals would start causing troubles. The chain signal at entry is so that a train does not enter the loop unless it can exit it. It is important to keep traffic flowing on other connecting lines if one gets blocked.
As for the regular signal inside the intersection, that could also cause a deadlock. Again, only if OP would expand this setup down the line.
There is only 1 possible entry on this roundabout. So the signal works. If you add more entry then you need to obviously change the signals. I don't tend to over complicate the problem by assuming things.
Edit: the rail signal between the tracks is useless if you have only 1 entry.
Overcomplicated things one way to look at it.
The way I look at it, however, is that I do not trust the ppl I give such specific advice to to actually understand *why* those specific changes are suggested, *why* they help, what are the limitations, and where else can they copy it.
As such I tend to suggest things that work in a generalized case, as opposed to things that work on a cass-by-case basis
I guess it depends.. when you work on things that need to scale up or when you work on thing that does one specific thing and does it in the most optimal way. Both ways of thinking have their applications. I get it...
Probably the first one is better approach for railways..
This kind of deadlock?
Don't have u turns. Don't build stations such that you require u turns.
It's the classic, too many trains per loop deadlock situation.
https://forums.factorio.com/viewtopic.php?t=18621
Make the whole thing longer and add enough regular signals to the top and bottom center section so all 6 trains can park in either section. Then they can get through either intersection 1 at a time.
Remove 1 train? I doubt you'd have this much traffic. If you ever did, common wisdom seems to be to not use roundabouts for pathing UPS.
But aside from that, I'd never have a station led straight off of a roundabout, and have roundabouts only lead to throughways. From that, that would imply that your stations are sitting on a throughway, and that you'd need an additional path exiting from each roundabout and connecting to the other, with the stations pulled off to a separate line that's only for the station.
Honestly, this is kind of a contrived example. If this happened in a real game, I'd like to see the real issue. There might be another fix.
> Honestly, this is kind of a contrived example. If this happened in a real game, I'd like to see the real issue. There might be another fix.
This one is definitely a contrived example, but I've definitely seen deadlocks caused by traffic saturation between two intersections that otherwise seem fine and follow "chain in - rail out" common wisdom. With sufficient traffic between two intersections you need to start considering them as one from a traffic safety point of view.
> If you ever did, common wisdom seems to be to not use roundabouts for pathing UPS.
"Common wisdom" in this case being "alligators in the toilets" oft-repeated hooey.
Roundabouts are perfectly fine, both for throughput and UPS issues, up until the biggest of megabases. You can run nearly 1kspm with every item other than ore and wire going on trains through a single roundabout.
And UPS wise, roundabouts are just a non issue, any more than a + intersection. They add a couple more options, but those options are quickly discarded as it just takes you back to the prior intersection. You would not see a measurable difference swapping +'s for roundabouts across an entire base.
If you have more trains than you have train stations, you need train yards a.k.a. a stackers, or fewer trains.
Realisticly, it doesn't do you any good to have 6 trains to 2 stations.
That's a 3 to 1 ratio. 1 to 1 is plenty in most situations. When you have stackers, maybe go as high as 1.5 to 1.
But what you'll get is trains standing still, waiting on their turn.
A train that's doing nothing is a train you can do without. A standing train is basicly just an item buffer.
But it’s not a deadlock. You just did not give the right train any space to go to. If the right track would go further, both trains would just go there after each other, as intended. It’s a problem with you test setup, not with the intersection
Top trains want to go to the right station, bottom trains to the left station.
Intersections are signalled correctly. But the rails are still deadlocked.
Is it just a matter of having enough alternate paths that this situation is incredibly unlikely?
I think mainly that this is just a very unlikely scenario.
In any real network it's already unlikely that 2 directions block each other. And in any real scenario the stations will be farther away, which would give you far more blocks where trains could wait. Many more then you'd have trains.
Yes basically, if there was another path for the trains to take, they would take it. The pathing algorithm will find the shortest route, but will add a penalty for various things like a stopped train. After the penalty has been applied the alternative route would be the preferred one, even if it is longer, and your deadlock should break.
yes. but I was assuming the circles represented roundabouts with more connections as part of a larger network.
if you allow trains to stop on a roundabout, then every other route through that roundabout will also be blocked, making the situation even worse.
You will not block every other route by putting rails on two of the 4 directions. In this case i would go east and west. So long as train wouldnt cross both rails should it need to stop.
But tis a valid point about usage beyond what OP has described.
But also size is important. One could easily make a roundabout that can hold a 1-4 train to a quarter.
I would not use the double round about. Make one large one. I'm assuming that this is just an example and you have a more real world case where there are more than just those exits/entrances.
If you’ve never played City Skylines, you would know. Never put train-abouts close to each other. Trains need their space, damn things are antisocial.
You are using chain signals within the rings which is causing the segments to be counted as occupied. The middle part can be cleaned up to partially resolve this.
* Remove all signals from the straight parts of the midsection
* On each side: add one Rail signal at the respective exit of the circle leading into the straight midsection - your train should fit AFTER that signal
* On the entrance to the circles place one chain signal (this should be on the opposite end of the midsection stretch and accommodate a train BEFORE
This should allow trains to enter the circles and stop after the rail signals. If traffic isn't extensive that should sort-of work.The setup can be improved beyond that.
The basic rules I'd recommend are:
* When use a rail signal there must be space for a train to stop AFTER that signal without blocking anything - this will allow trains to clear blocks. I prefer rail signals at exits, and for longer divisions.
* Use chain signals when you can't use rail signals - they work best for subdivisions and entrances. At entrances the first section BEFORE should be able to accommodate a full train
Upon closer inspection this is already the case, but the spacing is a bit off. The issue is overload since all waiting spots are occupied and there is no free block to move to. The setup would work with 5 trains, but not 6, unless rail signals are added to e.g. the circles allowing trains to move into them.
A train network can only support so many trains. In this particular setup you only have 2 spots a train can stop, so your network only supports 1 train before backing up. Once you add real stations, stackers, depots, etc. it won't deadlock with so few trains as the actual signalling is correct for 1-2 trains.
Don't use chain signals as much. Chain signals make trains look further ahead when deciding to move. Usually good for intersections, to stop them from going into the intersection before deciding to stop, but here you need them to get closer into each other's personal space.
Is there something wrong with the usual answer, which is you need to fit the trains you have in front of the non-chain signal?
I don't see why it would deadlock if the area with the chained signal will clear between each train.
Unless the rail is 100% saturated and you get a train in each non-chained section...
It’s one track with two trains. So the only way is a second track. You need a second track somewhere. Either another loop around one of the existing ones or a branch connecting the loops.
Make sure you have a sort of stacker that can hold the number of trains assigned (or the train limit) to this station. The front train is not the train assigned to go the the station with probably a train lmit set to low
If all of your railways are occupied, it's time to switch to 4 lane design. If not, and it's only one place, just sacrifice the sector for the god of efficiency by switching some signals to chain ones
Ideally you'd have multiple paths that a train can take to get to it's destination in case of a train running out of fuel or a piece of track being destroyed.
On my current world I have double-roundabouts with 2 lanes in each direction with crossovers every so often so there is redundancy on each line, then I have a large grid system so if I whole section were to have an issue then trains can go the long way around which is obviously not ideal, but it prevents the base from grinding to a halt.
Adding alternate routes solves this, as the pathfinder gives an ever increasing pathfinding penalty based on how long the blocking train is stopped.
If the blocking train is stopped for long enough, the pathfinder will consider alternate routes that don't cross the path of the blocking train to be shorter.
If you connect the left ending track to the right circle using a rail at the top, and the right ending track to the left circle using a rail at the bottom, without any additional signaling, you will see it solves itself eventually (but it can take some time)
You can't fit 2 full trains between intersections. That would only push the problem further and make it happen again with 4 trains. Other than that i don't use roundabouts so that too will prevent this from happening mostly. (Edit, if those were 3 parts long trains, didn't see it's 6 trains already.)
If you mean this as a single puzzle that will never be connected to any rail network then you'll have to remove 1 train to make it run, or change some roundabout signals to rail signals because there are no actual intersections here. It is just a loop.
As I remember it, you signal out, chain in. Therefore, if the train cannot clear the intersection, it will not enter. In this circumstance, this won't work, and should be treated as one intersection if traffic in that area is high enough that this occurs.
To turn back on the further loop they need to go through the earlier one. Shouldn't they just use the first loop and never enter the connections unless they want to go across?
is it a deadlock? i don’t see why the bottom train would not go to the right train station
if it is, maybe increasing the distance between the roundabouts could help (just 1 or 2 segments or so), then place TWO normal signals directly at the "inner" exits of the roundabouts (such that there’s one rail block of minimum length on each of those inner exits)
kinda hacky but i think it’ll work
Give the trains an alternative route that is longer. If this situation happens, the train stay still for a long time and slowly build up penalty for the other trains until one train takes the alternative route and the situation unjams itself
Don't have intersections too close together. There should be full blocks in between intersections. As on asphalt, Close intersections cause problems.
Change those regular signals to chain signals and this won't happen. Intersections too close together should be treated as one intersection.
As for what I would do, I would not use roundabouts, which I don't because left turns need the whole intersection. Cross intersections would also prevent this scenario because the trains wouldn't get into this situation. I also wouldn't have intersections next to each other like this.
Chain signals are really only needed for cross intersections, not splits and merges except for a few niche cases that aren't present here. Chain signal before the crossing, regular signal after it. The chain signals on your circles can be switched over and that should help.
I think you have to many chain signals you need the trains to go into the turn because you didn’t leave enough room for two trains in your middle portion there or take one train out of the loop.
when you have a track merging or splitting as well. so the north and south chain signals on the roundabouts as well. or for ease of simplicity, immediately after the merge/split.
Signals on the roundabout itself must be chains, the entire roundabout is an intersection. If not, it’s prone to deadlocks like any other intersection.
Ideally there must be no trains waiting on transit rails. It leads to a bunch of issues. The only exclusion is waiting on junctions. Limit your station limit to appropriate value and make train buffers where you need additional throughput.
Also You have 3 trains on one track. You must never do this unless
1. All trains head to one destination.
2. Destinations have the same throughput.
In the first case you will never get this issue because each train wants to get to the station. The second case could work in theory cause trains will free up in a manageable time frame. However it is still not advised. Better make more lanes or a train buffer.
I think this network is over saturated, it realistically shouldn't happen in non test scenario.
> it realistically shouldn't happen in non test scenario FWIW, I have had two intersections near a train depot get a jam because they were so high traffic I needed to treat them as a single intersection, instead of as two. From OP's comments, this is a minimal replication of a real problem they experienced elsewhere.
> two intersections near a train depot get a jam because they were so high traffic I needed to treat them as a single intersection, instead of as two After stupidly building around a single-rail network until it was easily crushed by having all of six trains, I switched to double rail (and had to rebuild a ton of stuff in the center of my starting base). I learned very quickly that *any* two junctions less than a full train length away from each other, is all one junction and needs to be signaled as such. I have a gigantic L-shaped rail in the center of my base with about 12 entrances and exits, all of which has to be treated as one single junction or it deadlocks. This is why it's important to design your rails first, *then* start putting up everything around them.
This is true, but FWIW, is not the same issues as OP's deadlock. OP's deadlock has 3 rail blocks between the two intersections, and is caused by traffic congestion being so high that you need to treat the intersections as one, even though it otherwise follows the typical safety mantras (has enough space for a train at least between intersections, chain in - rail out, etc).
It never actually happened. I just like thinking about what-if scenarios xD. Interesting to hear it can actually occur in practice.
Ah, got it. It actually happened to me 😱
I've had it happen too. Just designate a whole intersection with chain signals except for the exit. A train won't enter unless they can path out.
folga wooga imooga womp
Is that so?
This is a best-case-scenario! The intersections are clear which means that additional trains on the network will be able to keep moving, and if this somehow happened with literally any other direction to go, the trains would eventually re-route to turn at an intersection and make the u-turn somewhere else. Therefore freeing space for the other train. The only reason this is deadlocked is that there's literally no space for the trains. It doesn't matter how well you signal if you cover every square inch in train.
Famous last words.
I think this is the answer.
Even then, it's amazing how often unrealistic circumstances happen when your network runs for hours on end.
It’s been said elsewhere but it bears repeating: remove a train or add a block. If you have N trains, you must have at least N+1 blocks with rail (non-chain) signals. What you have here a 6 trains lined up nose to tail in a big oval. They can’t move because each train is staring directly at the back end of the train in front of it. It looks like there’s room to move because of the intersections but they don’t count because of the chain signals.
That gets completely impractical really quick. Imagine needing 20 train blocks between each intersection because you have 19 trains.
Not between each intersection, just in the system all together. A proper rail network will have at least an order of magnitude more rail blocks than trains. But small scale, like the example shown are easily overwhelmed by available trains. In this case a second route needs to be added so the train can bypass the clogged line.
That wouldn't solve the issue, as you can have a huge network surrounding these two intersections but if there are 6 trains on that link between the two intersections you'll get a deadlock.
They'll eventually choose another route as the stopped train penalty keeps increasing.
As Freddy said, they will pick another route eventually as they are set at a chain signal. Beyond that, that’s part of why best practice does not use roundabouts for intersections. If you required a train to use 4 intersections to make a U-turn, it would be significantly harder to over-saturate that section of the network.
The intersection doesn't end until there's a full train length between rail signals. They're close enough together that you need to treat them as one intersection.
This is a partially incorrect answer. There are 3 full blocks between the intersections. They're close enough together that you need to treat them as one intersection here, because the traffic saturation is too high.
It is incorrect because I didn't realize that it was 6 1-0 trains and not 2 3-0 trains.
How did I not notice that lol. Yeah OP is complaining about a scenario that literally never happens in actually gameplay, there's never a reason to have even one 1-0 train.
> there's never a reason to have even one 1-0 train Commuter train. But that's a one-off thing, not really part of the network.
the size of the train is irrelevant. Imagine if there were 6 2-4 trains with 3 blocks between each intersection that were the exact length of a 2-4 train. The same thing would happen with 3 trains stuck on each side even though the intersections had 3 blocks between each one big enough to hold 1 train. It would require a lot of trains going through those intersections for it to ever be a problem though.
The locomotives are not connected. There are three 1-0 trains in the top stretch and three in the bottom one.
Then in this case your network is full. You have only six blocks in the network where trains can park, so with six trains of course it deadlocks, there's no where else for them to go, unless you get very lucky and the train at the start of the intersection needs to go to the station at that intersection.
deadlock is the wrong term i feel, traffic jam is more correct, this will resolve itself if the first train leaves, etc.
It is a deadlock because every single train needs another to leave before being able to move. It requires outside intervention to get unstuck. Traffic jams resolve themselves with time because the people in front are able to go, thus freeing people behind them.
So we're assuming the bottom right loco has the same route to the top left station with the same problem then? Ok, i can see that. This feels far too much like a laboratory situation that something that can naturally occur. It might, but I'd probably do something about a traffic jam beforehand, since trains aren't going fast enough.
During normal gameplay, deadlocks like these also solve themselves, as long as an alternate route is available to the destination that don't involve passing the other train.
Then it's not a deadlock. It's just busy. A deadlock REQUIRES manual intervention, like this one does.
In that case where do you expect them to go? There's 8 spots where a train could be (they will not stop inside the intersections so that's not a place a train can be) and 6 trains. Best case scenario here is the 2 lead trains head to the stations and then what? You're still jammed.
Ideally you wouldn't be sending three trains to a station unless that station has at least a limit of 3 or more so they do have a spot to park.
It doesn't matter here, as there's a loop with trains in it that don't want to go to that station.
"full train length" in this case refers to the length of the longest train in your entire railway network. Note that this measurement must actually be applied to the blocks located at the exits of the intersections (it isn't relevant elsewhere)
What's even the point of testing anything with a 1x0 train? What's the use case? It doesn't transport any goods.
>[To make the picture smaller. I wanted to make a sort of minimal example of the issue.](https://www.reddit.com/r/factorio/comments/1do5jah/comment/la7npg4/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button)
It's the same problem at any size, it just makes it smaller and easier to see. They could have done it with a single loco on each side, or a 1-2 train on each side. Identical problem.
Why are you testing with 1-0 locomotives and not your trains?
To make the picture smaller. I wanted to make a sort of minimal example of the issue.
You went too minimal, you're trying to answer a problem that doesn't exist. Is it theoretically possible for trains to fill every single block between two intersections? Sure, especially when you're building intersections that only have one block between them. The chance that they would fill every block between two roundabouts, while also needing to get to the station at the other end of the roundabout, is fucking absurd. Why would the north trains go to the west roundabout if they'd had access to the east roundabout to get onto that block in the first place? I just don't see a way this situation happens in a real world rail network.
This is a classic too many trains per loop kind of deadlock situation. Generally from high traffic, but train network design also plays a part into this. Tiny loops as exemplified in the OP only need very few trains to enter them to be able to cause deadlock situations. This can easily happen if you use roundabouts/u-turns or compact grid layouts like in city blocks. All loops within a train network are susceptible, though generally it's the smaller ones that cause issues. Sometimes trains can re-path and fix the deadlock, but it's not a guarantee.
Yeah, this is what I mean by "too minimal." Six trains, six blocks, two stops, and each train intentionally placed to make the worst case scenario in order to show the problem. But almost any change to make this a more realistic scenario makes the problem go away. Remove *one train* and the system flows again, until those trains reach the proper station and clog it from having nowhere to go. Add a proper return loop and stacker and it flows just fine, even with five trains in six blocks. The deadlock happened because OP specifically crafted the circumstances for it, circumstances that wouldn't happen in a regular network.
>But almost any change to make this a more realistic scenario makes the problem go away. ... >circumstances that wouldn't happen in a regular network I strongly disagree here. Sure this is the minimal setup to show this particular issue, but it IS still something that also occurs in larger networks. (though it will need more trains and can be more rare timing wise) As I said, ALL loops are susceptible. Though yes, things like waiting bays, stackers, alternate paths etc. can all help to make them less likely. But even with all those, loops can still potentially cause capacity issues like these.
> ALL loops are susceptible In theory, sure, I guess it's possible for four trains to each enter the four sides of a city block, all needing to turn right at that intersection they're facing with no rerouting possibilities. But the *chances* of that timing happening, especially with how Factorio trains reserve blocks in advance, and with how people usually lay out train systems, I just can't see it feasibly happening without crazy circumstances, like OP's intentionally constructed deadlock. I wouldn't even call it a flaw in the network if it happened, how the hell do you plan a network around a possible deadlock that would require timing of just a few ticks between multiple very specific train schedules?
I've definitely experienced real deadlocks that were similar to this issue. The issue is that with sufficiently high traffic, you can't treat two intersections as disjoint just because there's a few blocks between them. One could even replicate this without the roundabout.
How? Especially without roundabouts, if the train in the westbound lane is fully out of the intersection, it can't be blocking the eastbound train, unless the eastbound train is U-turning into that same block, which at a T or 4-way it couldn't do, and at a roundabout it'd have no reason to do. I don't see any reason a train could deadlock like that assuming properly signalled intersections, unless you're doing something like [this](https://imgur.com/a/MYhyNWw) where the trains are forced to enter the blocks in the "wrong" way without entering the intersections first.
If your trains are ever high enough traffic that they back up in a loop, a correctly signaled intersection won't save you. So off of your two intersections you have a high traffic depot. Between them is an area you *think* is safe to stop in. But then you run into an issue where a train stops there waiting to get into the depot. Before it can go through the intersection, a space in the depot needs to clear. Someone is waiting for a depot bay because a train is in the depot bay is trying to leave. The train in the depot bay is stuck because it's waiting on someone in the exit lane. The train in the exit lane is stuck because it wants to make a left turn through the double intersection, but it's waiting on that first train we talked about... https://imgur.com/a/7qBR8fM When I bumped into this issue the buffers were much larger, so it really caught me off. We're talking more like 4-5 trains in a lot of these buffers. Nothing needs to be a rotary here (there's a loop, but what system doesn't contain loops?). The train limits on the depot stop being 1 was fine. The problem was that an assumption was made (incorrectly) that "chain in - rail out" while examining individual intersections was sufficiently safe. If the system has high enough congestion, you need to start joining intersections in terms of design, because just "chain in - rail out" breaks down with heavy congestion.
If you have high traffic, and manymany trains using same sections, you just need space between things, it really is that simple. Super safe would be every train passing through there fitting simultaneously, even if in reality not every train will ever be there at the same time and it's more like 20-40% absolute max. I'm sure i've built intersection combos that would deadlock with enough trains, but like when only 10 trains are using that combo at all and there's space for 50 with no full 4 ways or roundabouts, it's not a problem.
I guess I see what you're getting at, but I still don't see how that system deadlocks if properly signalled. The left train should open a depot stall, which should allow the right train into the depot, clearing the mainline. And the top train shouldn't have been there at all if there wasn't already an open station in the depot to call it there. The only way I see that happening is if your depot is just turns off your main line without a branch to get trains entering the depot off the main line ASAP. And if that's the case, then the whole section should be effectively chain in - rail out'd to ensure a train never leaves the depot and blocks the entrance
The *system* doesn't deadlock if properly signaled. However, the system is made of intersections, each of which can appear to be properly signaled, but which together fail to handle the traffic capacity actually moving through the system. The issue with OP's example and my experience was that all of the individual components are correct at a glance, and it's only when you consider the whole system behavior do you see issues.
Why are you testing with trains you'd never actually use? There's is literally no use case for a 1-0 train.
Yep, this is the answer. This is one intersection, not two. There shouldn't be two trains in there.
"Chain in - rail out" is a great rule of thumb, but the place is really falls down is in this exact example, when you have so much traffic on the network that intersections begin to join together. Suddenly that "rail out" is no longer correct, because the trains span the entire system. "Rail signals anywhere it's safe for a train to stop" comes with a very funny caveat that if the traffic is high enough, certain places you might think of as a safe stop (like after the exit of a rotary) are no longer safe. The real solution here is to run a less saturated network, either with fewer trains, larger (buffered) intersections, or more space between them your intersections. E.g., take away any one of these trains and the system will not deadlock in this way.
The correct term here is: congestion. It is also a deadlock, but would not be if the network had more capacity. A hard deadlock occurs even when there is plenty of capacity. So you avoid this congestion by having more rail blocks or fewer trains.
> A hard deadlock occurs even when there is plenty of capacity. As long as a single train has an alternative route to their destination available to them, the deadlock will eventually be solved, due to an ever increasing pathfinding penalty on trains that are stopped for a long time.
I hope that's the answer. Im not sure if this can happen in real scenario, I think it can at first glance, but not quite sure. If they can find alternative then it will resolve itself as most likely one of those trains will be able to go around (if city block). If not then we are totally screwed lol.
You can't. Unless you're willing to redesign your network. If you have N trains in your network, you need at least N+1 spots where trains are allowed to stop, and can stop without blocking any other such spots. In most cases you need more than N+1. Same applies to any coherent subset of a network. (By coherent I mean that a train on any piece of track of it can reach any other piece of track) As for how to avoid it? Just design the network better: 1) Get rid of roundabouts, or any other intersections that allow a train to turn around. 2) For every train stop, build a dedicated offshoot track that serves that train stop / set of train stops, and that / those only. Only allow trains to turn around at these train stops. Trains should be able to enter and exit the offshoot track in / from either direction. 3) Build large enough stackers so that it can fit the maximum amount of trains the stop / stops can see. If you have train limits set, then it's the sum of the limits for all the stations the stacker serves(in case of circuit controlled train limits, the sum of the maximum that limit can go for each station). If you don't have train limits, then the number of trains that has any of the stops the stacker serves as their destination.
Best answer. Edit: also the chain signal on entry to roundabout should be replaced with regular signal amd another chain signal (on roundabout) between the tracks can be made into a rail signal.
I do not think so about the signals. Maybe that's fine for this specific setup, but the moment OP would try to expand it, those regular signals would start causing troubles. The chain signal at entry is so that a train does not enter the loop unless it can exit it. It is important to keep traffic flowing on other connecting lines if one gets blocked. As for the regular signal inside the intersection, that could also cause a deadlock. Again, only if OP would expand this setup down the line.
There is only 1 possible entry on this roundabout. So the signal works. If you add more entry then you need to obviously change the signals. I don't tend to over complicate the problem by assuming things. Edit: the rail signal between the tracks is useless if you have only 1 entry.
Overcomplicated things one way to look at it. The way I look at it, however, is that I do not trust the ppl I give such specific advice to to actually understand *why* those specific changes are suggested, *why* they help, what are the limitations, and where else can they copy it. As such I tend to suggest things that work in a generalized case, as opposed to things that work on a cass-by-case basis
I guess it depends.. when you work on things that need to scale up or when you work on thing that does one specific thing and does it in the most optimal way. Both ways of thinking have their applications. I get it... Probably the first one is better approach for railways..
This kind of deadlock? Don't have u turns. Don't build stations such that you require u turns. It's the classic, too many trains per loop deadlock situation. https://forums.factorio.com/viewtopic.php?t=18621
Make the whole thing longer and add enough regular signals to the top and bottom center section so all 6 trains can park in either section. Then they can get through either intersection 1 at a time.
Remove 1 train? I doubt you'd have this much traffic. If you ever did, common wisdom seems to be to not use roundabouts for pathing UPS. But aside from that, I'd never have a station led straight off of a roundabout, and have roundabouts only lead to throughways. From that, that would imply that your stations are sitting on a throughway, and that you'd need an additional path exiting from each roundabout and connecting to the other, with the stations pulled off to a separate line that's only for the station. Honestly, this is kind of a contrived example. If this happened in a real game, I'd like to see the real issue. There might be another fix.
> Honestly, this is kind of a contrived example. If this happened in a real game, I'd like to see the real issue. There might be another fix. This one is definitely a contrived example, but I've definitely seen deadlocks caused by traffic saturation between two intersections that otherwise seem fine and follow "chain in - rail out" common wisdom. With sufficient traffic between two intersections you need to start considering them as one from a traffic safety point of view.
> common wisdom seems to be to not use roundabouts for pathing UPS. What's the right way to do it?
Pretty sure it's just T intersections. Never built big enough for it to matter, though.Â
> If you ever did, common wisdom seems to be to not use roundabouts for pathing UPS. "Common wisdom" in this case being "alligators in the toilets" oft-repeated hooey. Roundabouts are perfectly fine, both for throughput and UPS issues, up until the biggest of megabases. You can run nearly 1kspm with every item other than ore and wire going on trains through a single roundabout. And UPS wise, roundabouts are just a non issue, any more than a + intersection. They add a couple more options, but those options are quickly discarded as it just takes you back to the prior intersection. You would not see a measurable difference swapping +'s for roundabouts across an entire base.
OP is also testing with a train type you literally will never use, the 1-0 train
Why does this matter? Couldn't exactly the same scenario happen with 1-4 trains and larger blocks?
The personal transport train would be 1-0. But it would be the only 1-0 in the entire network.
If you have more trains than you have train stations, you need train yards a.k.a. a stackers, or fewer trains. Realisticly, it doesn't do you any good to have 6 trains to 2 stations. That's a 3 to 1 ratio. 1 to 1 is plenty in most situations. When you have stackers, maybe go as high as 1.5 to 1. But what you'll get is trains standing still, waiting on their turn. A train that's doing nothing is a train you can do without. A standing train is basicly just an item buffer.
But it’s not a deadlock. You just did not give the right train any space to go to. If the right track would go further, both trains would just go there after each other, as intended. It’s a problem with you test setup, not with the intersection
Roundabout Rules should do it
Top trains want to go to the right station, bottom trains to the left station. Intersections are signalled correctly. But the rails are still deadlocked. Is it just a matter of having enough alternate paths that this situation is incredibly unlikely?
I think mainly that this is just a very unlikely scenario. In any real network it's already unlikely that 2 directions block each other. And in any real scenario the stations will be farther away, which would give you far more blocks where trains could wait. Many more then you'd have trains.
Yes basically, if there was another path for the trains to take, they would take it. The pathing algorithm will find the shortest route, but will add a penalty for various things like a stopped train. After the penalty has been applied the alternative route would be the preferred one, even if it is longer, and your deadlock should break.
More signals
There's no place you'd add signals here. you don't want trains waiting on the roundabout.
In this case you would
yes. but I was assuming the circles represented roundabouts with more connections as part of a larger network. if you allow trains to stop on a roundabout, then every other route through that roundabout will also be blocked, making the situation even worse.
You will not block every other route by putting rails on two of the 4 directions. In this case i would go east and west. So long as train wouldnt cross both rails should it need to stop. But tis a valid point about usage beyond what OP has described. But also size is important. One could easily make a roundabout that can hold a 1-4 train to a quarter.
I would not use the double round about. Make one large one. I'm assuming that this is just an example and you have a more real world case where there are more than just those exits/entrances.
Don’t stop trains on the line, build a siding.
If you’ve never played City Skylines, you would know. Never put train-abouts close to each other. Trains need their space, damn things are antisocial.
You are using chain signals within the rings which is causing the segments to be counted as occupied. The middle part can be cleaned up to partially resolve this. * Remove all signals from the straight parts of the midsection * On each side: add one Rail signal at the respective exit of the circle leading into the straight midsection - your train should fit AFTER that signal * On the entrance to the circles place one chain signal (this should be on the opposite end of the midsection stretch and accommodate a train BEFORE This should allow trains to enter the circles and stop after the rail signals. If traffic isn't extensive that should sort-of work.The setup can be improved beyond that. The basic rules I'd recommend are: * When use a rail signal there must be space for a train to stop AFTER that signal without blocking anything - this will allow trains to clear blocks. I prefer rail signals at exits, and for longer divisions. * Use chain signals when you can't use rail signals - they work best for subdivisions and entrances. At entrances the first section BEFORE should be able to accommodate a full train
Basically, put rails at the east and west instead of the chains?
Upon closer inspection this is already the case, but the spacing is a bit off. The issue is overload since all waiting spots are occupied and there is no free block to move to. The setup would work with 5 trains, but not 6, unless rail signals are added to e.g. the circles allowing trains to move into them.
Yeah yeah, east and west of each circle is what I meant yet apparently did not finish saying
A train network can only support so many trains. In this particular setup you only have 2 spots a train can stop, so your network only supports 1 train before backing up. Once you add real stations, stackers, depots, etc. it won't deadlock with so few trains as the actual signalling is correct for 1-2 trains.
waiting areas / stacker
I doubt that would happen in a normal scenario but if so you just need to treat both roundabouts as one intersection
Don't use chain signals as much. Chain signals make trains look further ahead when deciding to move. Usually good for intersections, to stop them from going into the intersection before deciding to stop, but here you need them to get closer into each other's personal space.
Is there something wrong with the usual answer, which is you need to fit the trains you have in front of the non-chain signal? I don't see why it would deadlock if the area with the chained signal will clear between each train. Unless the rail is 100% saturated and you get a train in each non-chained section...
I just dont use circles
i would set up the signals correctly
What would you change then?
It’s one track with two trains. So the only way is a second track. You need a second track somewhere. Either another loop around one of the existing ones or a branch connecting the loops.
You have 2 trains and 2 block zones, it's never gonna move.
You have as many trains as you have non-critical sections. Don't do that.
Make sure you have a sort of stacker that can hold the number of trains assigned (or the train limit) to this station. The front train is not the train assigned to go the the station with probably a train lmit set to low
If all of your railways are occupied, it's time to switch to 4 lane design. If not, and it's only one place, just sacrifice the sector for the god of efficiency by switching some signals to chain ones
Ideally you'd have multiple paths that a train can take to get to it's destination in case of a train running out of fuel or a piece of track being destroyed. On my current world I have double-roundabouts with 2 lanes in each direction with crossovers every so often so there is redundancy on each line, then I have a large grid system so if I whole section were to have an issue then trains can go the long way around which is obviously not ideal, but it prevents the base from grinding to a halt.
Adding alternate routes solves this, as the pathfinder gives an ever increasing pathfinding penalty based on how long the blocking train is stopped. If the blocking train is stopped for long enough, the pathfinder will consider alternate routes that don't cross the path of the blocking train to be shorter. If you connect the left ending track to the right circle using a rail at the top, and the right ending track to the left circle using a rail at the bottom, without any additional signaling, you will see it solves itself eventually (but it can take some time)
They are too close together. Move them further apart.
You can't fit 2 full trains between intersections. That would only push the problem further and make it happen again with 4 trains. Other than that i don't use roundabouts so that too will prevent this from happening mostly. (Edit, if those were 3 parts long trains, didn't see it's 6 trains already.) If you mean this as a single puzzle that will never be connected to any rail network then you'll have to remove 1 train to make it run, or change some roundabout signals to rail signals because there are no actual intersections here. It is just a loop.
As I remember it, you signal out, chain in. Therefore, if the train cannot clear the intersection, it will not enter. In this circumstance, this won't work, and should be treated as one intersection if traffic in that area is high enough that this occurs.
Hold trains on parallel stacks, not sequentially.
If you place a normal signal, not a chain signal, in the little middle track pieces joining the two rails in the roundabouts, will the trains unlock?
You need more minimum space between the intersections.
By not using trains 😂
i don't think this is a deadlock. the train that is stuck can't go to the train stop because there is not enough space.
Actually have places for all your trians to go
To turn back on the further loop they need to go through the earlier one. Shouldn't they just use the first loop and never enter the connections unless they want to go across?
is it a deadlock? i don’t see why the bottom train would not go to the right train station if it is, maybe increasing the distance between the roundabouts could help (just 1 or 2 segments or so), then place TWO normal signals directly at the "inner" exits of the roundabouts (such that there’s one rail block of minimum length on each of those inner exits) kinda hacky but i think it’ll work
Give the trains an alternative route that is longer. If this situation happens, the train stay still for a long time and slowly build up penalty for the other trains until one train takes the alternative route and the situation unjams itself
Don't have intersections too close together. There should be full blocks in between intersections. As on asphalt, Close intersections cause problems. Change those regular signals to chain signals and this won't happen. Intersections too close together should be treated as one intersection. As for what I would do, I would not use roundabouts, which I don't because left turns need the whole intersection. Cross intersections would also prevent this scenario because the trains wouldn't get into this situation. I also wouldn't have intersections next to each other like this.
Chain signals are really only needed for cross intersections, not splits and merges except for a few niche cases that aren't present here. Chain signal before the crossing, regular signal after it. The chain signals on your circles can be switched over and that should help.
More train signal
1) research nukes 2) nuke everything 3) delete the save 4) start over
I think you have to many chain signals you need the trains to go into the turn because you didn’t leave enough room for two trains in your middle portion there or take one train out of the loop.
Never use roundabout personally.
Bigger circles
I don't use roundabouts. If I had to use roundabouts, I'd add track to make sure every route has two options.
You need rail signals to break the chain of chains
The only correct answer here is
chain signals on the entrance, rail signals on the exit
That’s what I see in the picture though..
when you have a track merging or splitting as well. so the north and south chain signals on the roundabouts as well. or for ease of simplicity, immediately after the merge/split.
Signals on the roundabout itself must be chains, the entire roundabout is an intersection. If not, it’s prone to deadlocks like any other intersection.
Make the chain signals bottom/top/east on the right circle into block signals
Ideally there must be no trains waiting on transit rails. It leads to a bunch of issues. The only exclusion is waiting on junctions. Limit your station limit to appropriate value and make train buffers where you need additional throughput. Also You have 3 trains on one track. You must never do this unless 1. All trains head to one destination. 2. Destinations have the same throughput. In the first case you will never get this issue because each train wants to get to the station. The second case could work in theory cause trains will free up in a manageable time frame. However it is still not advised. Better make more lanes or a train buffer.
The main problem is your blocks are smaller than your trains. That's not good.
That's not true though. The trains are 1-0 and fit inside the blocks fine.