first this happened:

trains stopped at a signal head on. they would not reverse, nor could they find the way to the depot.
one thing worked: assigning one train to another line. i thought maybe it reverses then. it did, but also this happened:

the trains passed right through each other. … i was not sure if i was happy about this or not.
This here actually documents a multituded of bugs or design flaws:
- trains can only be sent to a depot, if they can go there by going forward. if they would need to reverse to get there, they are lost.
- trains can get stuck head on at a signal. and without a lucky workaround (which actually may only be possible due to yet ANOTHER bug) they may be stuck there until the player quits and loads the last savegame.
- when a train is assigned to a line, while it is out on the tracks, all kind of weired shit may happen. i am pretty sure of that. the passing through bug seems to be a result of the train assignment and not of the getting stuck head on bug.