Basically there are two possible reasons why a train can not reach a destination:
- There is no physical link between the trains current position and the destination, i.e. tracks are missing.
- There are tracks, but the signals are placed in a way that prevents trains from going through in a certain direction.
In order to keep it short I suggest “No Tracks” or “Tracks missing” for a) and “Path Blocked” for b).
Additionally it might be a good idea to add a global alert for trains that are stuck, to help detect deadlocks. (You know, one of those annoying yellow triangles that flash every other second once you start using robots)