In signaling, the section of track on the approach side of a signal which is equipped with a circuit to detect the arrival of a train and transmit its presence to the controlling circuits of the signal and its associated route. Used to lock a route and prevent it from being altered once a train has approached within a safe braking distance, known as approach control. This prevents the route being changed at a time when the train could run onto it and be derailed. Also use to clear signals normally maintained at danger until a train has approached within a given distance. This distance is calculated to ensure the locomotive engineer sees a red signal as he approaches. This has the effect of causing the locomotive engineer to reduce train speed to a required level, at which point the signal will clear.