Interlocking

In railway signalling, an interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as junctions or crossings. In North America, a set of signalling appliances and tracks interlocked together are sometimes collectively referred to as an interlocking plant or just as an interlocking, and the term "interlocking" is sometimes used to refer to a set of railroad switches that allows trains to move from either of two tracks to the other. An interlocking system is designed so that it is impossible to display a signal to proceed unless the route to be used is proven safe.

Interlocking is a safety measure designed to prevent signals and points (switches) from being changed in an improper sequence. For example, interlocking would prevent a signal from being changed to indicate a diverging route unless the corresponding points had been changed first. In North America, the official railroad definition of interlocking is: "An arrangement of signals and signal appliances so interconnected that their movements must succeed each other in proper sequence".