There may be several answers to this problem, @lenscab.
This is mine: I happened to have the same phenomenon, an AI-driven train refusing to continue beyond a certain point for no good reason. I repeated the session, with Message Overlay on, and up came the message: cannot plan ahead, junction missing lever.
Either the junction that your train is stopping at has lost its lever (or never had one), or another junction further down the line is lacking one. Assuming it is an editable route you have, check for missing levers, go into Surveyor and add the lever. Save and try the session again.
But as I said, there may be other answers - better ones, even.
Good luck.