This is a bug in the game. Sometimes trains lose their destinations when you start reorganizing their trackage.
The workaround is to:
– stop the train ( click on the “running” status )
– reverse the train
– start the train
repeat if the train is facing incorrectly ( against the ribbon of its line )
If that does not work, point the train into a depot ( build one if you need to ) and send it there. Then reassign a line.
Generally speaking. In such case you need to fiddle with the train controls and/or reassign the line to such train.