The Automatic Train Supervision sub-system provides for headway management and proper interleaving of trains at junctions by speeding up or slowing down trains. The system works via a series of track mounted balises, which function as information beacons transmitting a speed code to the train as it passes overhead. Its designers intended its function to be non-vital, and for this reason, the design is not fail-safe. At no point can the ATC system override the maximum speed limit enforced by the cab signal system.