An Entrance/Exit (or short “Extern”) in CTC is a point where trains enter or leave your territory based on their schedule. These are points adjacent to your territory controlled by another dispatcher, be it at a desk next to you, working at a different subdivision, or even another railroad company. Each can have one or more tracks (“extern track”) with a name applied to all tracks within the extern group. Each track has a direction – inbound or outbound – showing the current state of permission coordinated with your counterpart who can send or receive a train at a given time.
The schedule determines which exits a train has to use in order to leave the territory on its current leg. If you send a train on a route that makes the destination unreachable, the train driver will notice that, stop the train, and give you a call.
To complete the destination, any track within the access group can be used.
If a train is involved in an incident that would not allow it to complete its schedule, it is deemed ”Out of Service”. Such a train can be sent to any exit.