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I have over 500 hours in this game and have never had a signaling problem that I couldn't solve until now.

I have this collision and I can't figure out why:


Here is a view without (most of) the trains:


I can't see a signaling mistake. Most of the outer track, from the right, down along the bottom, and up the left side is one block.

As far as I can tell, the train with the logs caused the collision and, if I'm correct, that train came from the bottom track, the one track without a signal leading on to the junction area.
My guess is that when you add a signal to that track (just like with the rails directly above it), your problem will be solved.

The track at the bottom is intended to be part of the same block. You can see on the right side that all of the access to that track is marked with chain signals. Sorry if it's a bit confusing. I could add signals there, but it shouldn't need them as far as I can tell, so I want to understand what is happening.

Does it happen repeatedly? Because if it doesn't, this could just be a coincidence triggered by something as small as replacing a signal or adding/removing a piece of track somewhere.

Yes, it's happened several times. The first time I just ignored it and hoped it didn't happen again. The second time I was busy with other stuff and didn't feel like trying to figure it out. I think this is the third time now. It's definitely a reoccurring issue. It's always happened off-screen, and not at any time when I was messing with the signals near this junction. I don't think a temporary no-path error should cause this should it? If the copper train just stops because of a no-path it would cause a deadlock but not a collision. The fuel train should still not enter the block. I'm stumped.

There could be several issues to be honest. One thing that immediately caught my eye was the fact that your 'large block' also includes all the junctions.
Keep in mind that when a chain signal has to judge heaps of other signals to make its decision, things get complicated real fast. Your trains might even try to bypass the route that you intended for them,
and use a shorter route through a station or something.

So, that train with the logs, where is the signal that should have told it to wait before driving onto that large block with all the junctions?
-edit Is that where, in the top screenshot, the black train is waiting on the right side, next to the mini map?

Yeah, sometimes trains will path through empty stations and that should be fine; as far as I can tell it's all signaled properly to allow for that. You can see in the second screenshot all the possible routes to access that block. I don't know specifically from which one the fuel train came. There are four possibilities. From the outside connection on the left it would have come through the top block (signal on the top right). It could also have come from the fuel station (at the top), the outside connection on the right (this is where the black train you mention is), or it might have come through the farl station (second track from the bottom) because the farl train was out with me. But I don't see any problems with any of the possible paths.

No, I agree, I don't see an immediate problem either, but they can hide in corners we know nothing about.
What I'm trying to say is that when chain signals look at the signal ahead, they often have to judge multiple and if I'm honest, I don't know the exact mechanics behind that to explain what is going wrong.
I can only tell you how to solve it, and from the looks of it, things can be solved by separating the very bottom track from the large block.
A regular signal at the entrance and a chain signal at the exit, just like at the sections with your stations will probably help.

Ok, I'll try adding signals there as well. Might just be a bug. Thanks for the help.

Unless you're adding trains or signals or whatever collisions shouldn't be possible. A collision has to happen when two trains are in the same block which signals don't allow.

I doubt it's a bug (there used to be one where trains crept through chain signals but that's been fixed), is it possible something like this happened:

• Train parks at one of the stations
• You adjust something around the station, a second train is allowed into the second station, no collision
• At some point, one of the trains moves up to the signal
• Some time after that, something changes to trigger the second train moving up to the signal
• Trains collide

This is the only thing I can think of that would cause this, and would explain why you weren't there when the collision happened.

Sorry, I don't follow your example. Can add more specifics? Exactly which stations you mean, which signals, and what kind of "adjustments" might cause this? All the relevant signals and stations are visible on my screenshot.

Ok, now I have this collision! Again when I was far off-screen. WTF factorio. Does anyone have any idea what is happening?