I just got a reply from a beta tester on Steam:
“The lines are fluid – The train will simply pick another platform to stop at and try to get back onto the original line whenever it can through switch tracks. 🙂
You can’t tell a train to always use Platform 3 with how the game is right now. They’ll just pick whichever is available and closest to the original line design.”
Too bad there are different views and ideas on this, but I hope he’s right :p