This started with the latest update, which included "network trains will no longer travel partially loaded when the supply is bottlenecked, and instead wait for their full reservation".

Well, now I've got a train waiting for its full reservation... when all cars for its current alignment are full and it needs to move to the next, but it's refusing to, so it's just going to be waiting forever.

Oh, and the supply on this station is not bottlenecked. Loads four cars at once, that's manufactured sand on the left and raw rock on the right, but gravel (which the train is picking a full load of) in the middle.

Not sure how to reproduce it, but as I just had it happen multiple times in parallel (at this very station, look ~3 trains back in line for one half full of diesel, already ejected from its loading position), perhaps it has something to do with multiple trains loading different products from the same station simultaneously, which was made practical by the advent of the Train Network? Affected save file is attached, should be in the pictured position, and I confirmed the issue persists through reload.

Update 4.1 | v0.8.4 (b587)

Come to think of it, I have noticed that adding modules to the front of a station while a train is actively working at the station will confuse its later alignment positions- like they're only searched when it first arrives, and are specifically relative to the far end of the station from where it entered, rather than the actual station unit of which there can only be one. Adding or removing modules behind a train has no notable effect. Fairly easy workaround, don't extend/shorten stations while trains are loading/unloading (on route is OK, they just can't be in "At station" or "Arriving" status)... but it makes me wonder if the above bug happened because it somehow forgot that the next alignment position existed, and failed to check for missing positions when it exhausted the list?

I should probably note that while the station this issue arose on does provide fuel to the trains, I did not check if it had run out or not- but this train was sitting for way too long to have been waiting for fuel. Not only that, but once this train was cleared (by cancelling its job), the trains stuck behind it loaded and shipped out quickly, as expected.

Can confirm, building a module while the train is loading\unloading triggers the issue.

The issue in the original report was not caused by station modification; most recent of that was decades ago, possibly centuries, by game time. But station modification would cause it, or at least something similar, thanks to the alignment confusement effect.

Interesting. It seems to be happening precisely once per play session on that save, within half an hour or so of launching the game... but never involving trains that were already at the station when the world was loaded. And always on that one station, never any of the others.

Okay... just had it happen to 5 trains at once, again at that one station. It looks like they're getting stuck waiting for fuel, but failing to draw fuel from the plentiful supply in the station. Don't know why, are the 8 fuel modules getting distracted by another train and never returning?

Also, can we have a toggle for whether trains are allowed to wait for fueling at a station (current behavior and expected at designated fuel stops), or only allowed to fuel for as long as they're transferring other cargo (which I want at these large stations, to prevent trains from waiting for fuel at a busy cargo station, esp. when there's a shortage, but still want them to get some so they aren't constantly running back to the dedicated fuel station).

Confirmed fueling theory, Unity-refueling also gets stuck trains to move on. That means the bug wasn't what I initially thought it was, and I ought to change the topic name...

... Interesting. If multiple trains are stuck at once, unity-refueling will only progress it to the next position, where it'll promptly re-stuck itself. If it's the only/last one, though, it'll finish the pickup/dropoff properly.

And freeing the frontmost train seems to free the rest automatically... had that one stick in its final alignment position rather than its first...

Thanks. You're right: it was an issue with fueling. There is a bug whereby if multiple trains are trying to refuel at the same station at the same time with the same type of fuel, none will get refueled.

Should be fixed in an upcoming patch.

Thank you! I suppose that means that in those cases where only one train was affected, there was another affected train in the station that I just hadn't noticed yet, and which was "auto-released" by the release of the first. Which makes sense, given the details I've observed lately.

Interesting. I didn't see anything about this issue on the changelog, but it appears to be fixed in v0.8.4a (b588). Been running for several hours and I haven't had it happen once, even though that station has only gotten busier.

Correct, it's in the current experimental version.