A railway interlocking is the safety system that controls points and signals so that a train can only be given a route that is set, locked and free of conflicting movements. It is the core of signalling at stations and junctions, and every route requested by a signaller or a traffic management system must pass through it.
An interlocking checks that the points are in the right position and locked, that the track sections are clear and that no conflicting route is set before it clears a signal. Track circuits or axle counters tell it which sections are occupied. The route stays locked until the train has passed.
Without the interlocking, collision protection at junctions would depend on the signaller’s attention alone. Interlockings are developed to Safety Integrity Level 4, the highest level in the CENELEC railway safety standards.
From levers to digital interlocking
The first interlockings were mechanical, with levers in a signal box connected by rods and wires. Relay interlockings replaced them from the mid-20th century, and computer-based interlockings followed from the 1980s.
An interlocking also provides the safety basis for ETCS Level 2. The Radio Block Centre builds movement authorities from the routes the interlocking has set and locked.
Digital interlockings move the interlocking computer to a central site and control points, signals and train detection over standard data networks. That removes much of the dedicated signalling cable along the line.
Interlockings and field elements have traditionally used proprietary interfaces, so equipment from different suppliers does not work together. EULYNX, a consortium of European infrastructure managers, standardises the interfaces between the interlocking and the field elements. The aim is to let infrastructure managers buy components from different suppliers and replace them separately over their life cycle.