Editing Interlocking (section)

== Configuration and use ==
[[File:Pennsylvania Railroad, HOLMES Block Station modelboard and lever machine.jpg|thumb|A model board and lever machine]]

A minimal interlocking consists of [[Railway signal|signals]], but usually includes additional appliances such as [[Railroad switch|points]] and Facing Point locks (US: switches) and [[Derail (railroad)|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** the signal to proceed is withdrawn and sufficient time has passed to ensure that a train approaching that route has had opportunity to come to a stop before passing the signal.