RailFanning is a gift you give yourself. - Varun

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.

A minimal interlocking consists of signals, but usually includes additional appliances like switches / Points, derails, crossings at grade and movable bridges. Some of the fundamental principles of interlocking include:
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1. Signals may not be operated to permit conflicting train movements to take place at the same time.

2. Switches and other appliances in the route must be properly 'set' (in position) before a signal may allow train movements to enter that route.

3. 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
a. The train passes out of the portion of the route affected, or
b. The signal to proceed is withdrawn and sufficient time has passed to ensure that a train approaching that train has had opportunity to come to a stop before passing the signal.