I do not want to use "routing". Can you deal? Thank you, Kuba
So your trains are just randomly wandering around the map?
I think he meant he don't want to use waypoints or signal contraptions.
The problem there might be how the network is built, if a train for any reason (wrong double signal at a station, passing point, split/join etc) takes the wrong track and can't return at the starting point because there is no valid path, then it gets lost.
Or even a weird path, like a T where you need to go on the bottom leg if you want to go from top left to top right.
With such little information provided, lacking better screenshot of the problem, savegame, pathfinder used, penalties, I can't tell where the exact problem might be...