So - my rail system was working for a long time, and I was off on the other side of the map adding a new station, when I got a message that objects got destroyed. Looking at the info - it showed that a locomotive and a few cargo wagons were destroyed, without being attacked first - rail crash.
I reloaded the last autosave and rushed back to the main station, where I found this:
Look at the leftmost diagonal rail - the train in front was there for a long time, not programmed to go anywhere, and this setup has worked for a long time. As you can see - there is a signal before it, so the train at the back must have run it while it was red, thus getting stuck and now showing "No Path".
At this stage I wondered - I got here after the train had run the red light, so how come I didn't see a crash? As I mentioned - I have reloaded the autosave and rushed back quickly, so I didn't place down another signal I did place in the first run. As I mentioned - this was far far away, and shouldn't have made a difference, but apparently - it did!
So - my guess, and it is a wild guess, I admit, is that somehow placing a signal at a remote location at the exact right moment changed the block numbers while a train was trying to figure if it can or cannot enter a certain block, thus causing a crash.
I would upload a saved game, but I have explored a lot in this game, and the file is already at 35MB
I tried reproducing it in a new (smaller) game, but couldn't. Perhaps it really is a matter of exact timing.
