Mechanical railway signalling installations rely on lever frames for their operation to interlock the signals, track locks and points to allow the safe operation of trains in the area the signals control. Usually located in the signal box, the levers are operated either by the signalman or the pointsman.

On a rail transport system, signalling control is the process by which control is exercised over train movements by way of railway signals and block systems to ensure that trains operate safely, over the correct route and to the proper timetable. Signalling control was originally exercised via a decentralised network of control points that were known by a variety of names including signal box, interlocking tower and signal cabin. Currently these decentralised systems are being consolidated into wide scale signalling centres or dispatch offices. Whatever the form, signalling control provides an interface between the human signal operator and the lineside signalling equipment. The technical apparatus used to control switches (points), signals and block systems is called interlocking.