Summary
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. The signalling appliances and tracks are sometimes collectively referred to as an interlocking plant. An interlocking 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/switches 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". A minimal interlocking consists of signals, but usually includes additional appliances such as points and Facing Point locks (US: switches) and derails, and may include crossings at grade and movable bridges. Some of the fundamental principles of interlocking include: Signals may not be operated to permit conflicting train movements to take place at the same time on set route. Switches and other appliances in the route must be properly 'set' (in position) before a signal may allow train movements to enter that route. Once a route is set and a train is given a signal to proceed over that route, all switches and other movable appliances in the route are locked in position until either the train passes out of the portion of the route affected, or the signal to proceed is withdrawn and sufficient time has passed to ensure that a train approaching that route has had opportunity to come to a stop before passing the signal. Railway interlocking is of British origin, where numerous patents were granted. In June 1856, John Saxby received the first patent for interlocking switches and signals.
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