Interlocking

For the use of the term interlocking in music, see Hocket In railway signaling, an interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as junctions, crossings, and so forth. The signaling appliances and tracks are sometimes collectively referred to as an interlocking plant. An interlocking is designed so that it is impossible to give clear signals to trains unless the route to be used is proved to be safe.

A typical 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 like switches (points in UK parlance), derails, 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.
Switches and other appliances in the route must be properly lined up (in position) before a signal may allow train movements to enter that route.
Once a route is lined up 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- the signal to proceed is withdrawn and sufficient time has passed to ensure that a train approaching that signal has had opportunity to come to a stop before passing the signal.

The bars are constructed so that, if the function controlled by a given lever conflicts with that controlled by another lever, mechanical interference is set up in the cross locking between the two bars, in turn preventing the conflicting lever movement from being made.

Interlockings controlled electrically consist of complex circuitry made up of relays that ascertain the state or position of each signal appliance. As appliances are operated, their change of position opens some circuits that lock out other appliances that have would conflict with the new position. Similarly, other circuits are closed when the appliances they control become safe to operate. Equipment used for railroad signaling tends to be expensive because of its specialized nature and fail-safe design.

Modern interlockings — those installed since the early 1990s — are generally solid state, where the complex wired networks of relays are replaced by software logic running on special-purpose fail-safe control hardware. The fact that the logic is implemented by software rather than hard-wired circuitry greatly facilitates the ability to make modifications when needed by reprogramming rather than rewiring.

By whatever means of technology are used, interlockings are designed to ensure that no operation can be performed unless all prerequisites have been satisfied.

Interlockings operated solely by electrical circuitry may be operated locally or remotely. Furthermore, such an interlocking may be designed to operate without a human operator. These arrangements are termed automatic interlockings, and the approach of a train sets its own route automatically, provided no conflicting movements are in progress.

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