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I've occasionally noticed situations where my 2 locomotive + 8 cargo wagons trains are crashing, and I eventually isolated it to cases where they crash with themselves. I've attached a save file from ~5 seconds before this happens, and can include additional saves up to about 3 minutes in advance. If you watch the train routing, several changes in destination stations turning on/off causes this train to enter a roundabout from the east side, loop a full 360 degrees, and then exit north, which snags the last train car.

Because this issue is triggered by minute changes in train station switching in a megabase, I haven't been able to reduce the problem to a minimal game save. You can download the save from Google Drive.

To be fair I'm not even sure if this is a bug. Ie. should train routing prevent a train from colliding with itself? Is that even possible? Anyway, I'm submitting this just in case this is an actual (and fixable) bug. If this is working-as-intended, I'd like to hear suggestions on how I can improve my train routing to fix this issue, though maybe that belongs in a different thread.

Unfortunately I think this is a dupe of 77759 which is filed as "Not a bug."

Yeah I couldn't really think of how the pathing algorithm would account for this, unless the roundabout was split into different regions. Any suggestions on how to change the signaling to prevent this? I'm not sure if there's enough space to increase the roundabout size. Or I could resign myself to 1x8 trains, which would be a different major redesign.

I unfortunately also don't have any sage advice for you.

This however might be good to bring up in any of the following locations were more seasoned engineers are likely to lurk:

• Gameplay Help forum
• The community subreddit
• The Factorio Discord

If you are prepared to live without the right turn bypass, there should be plenty of room to expand the roundabout. (Looking at it I think there probably is enough room even without touching the by-pass. Just start the first turn 1 track-piece earlier).

Also there are plenty of other designs for intersections in viewtopic.php?f=194&t=46855 . Chunk aligned cross is pretty much the same size, with similar performance, but won't have the self collision issue.