Overlap (railway signalling)

In railway signalling, overlap refers to the distance beyond a signal at danger that a train is required to keep clear. It is a critical safety margin designed to prevent collisions in situations where a train is unable to stop before reaching the signal.

The overlap distance provides a buffer zone, ensuring that even if a train overruns a signal indicating danger, it has a reasonable chance of stopping before conflicting with another train movement (such as a train on a converging track, or an obstruction on the line ahead). The length of the overlap is carefully calculated, taking into account factors such as the maximum permitted speed of trains on the route, the gradient of the track, the braking characteristics of the trains, and prevailing weather conditions that might affect braking distance.

The precise implementation and length of the overlap varies depending on the specific signalling system, the operating rules of the railway, and national or regional regulations. The presence of an adequate overlap is a fundamental requirement for safe train operation, and any compromise or reduction in the overlap distance can significantly increase the risk of accidents.