Hi,
This is a good question. But arcgis does only static modeling if I am not wrong. The case you suggest would need coding. a time window option might help, but it is still static. following a procedure of 1, -1, 0 , <>, or TF FT N, In coding, you could possibly make the line N indicating no access, and therefore, the second train cannot find a path, for the required time steps. or secondly this could mean that the train would have a 'moving' origin and destination for each step, 'along' the route till they reach the final destination from the initial origin. this will call shortest path routine in each step with the moving origin and destination.
regards,