Points and interlocking

Its job in the system

Points, also called switches or turnouts, let a train pass from one track to another. At a junction they connect routes that may compete for the same physical space. A train cannot take both routes at once, and two trains cannot safely cross the same conflicting path together. This makes route setting a coordination problem as well as a mechanical one.

How it works

The movable rails must be in the required position for the selected route. Detection and locking arrangements establish the relevant conditions before a movement is authorised. An interlocking links these conditions to signals and route permissions, preventing incompatible authorisations. The principle is a set of safety relationships; it can be implemented with different generations of equipment.

Junctions consume time as well as space. A route remains unavailable to a conflicting movement while the first train passes and until the required conditions for release are satisfied. A merge between two frequent branches can therefore constrain the shared section even when each branch has room for more trains. Changing the order of trains changes who waits.

What it connects to

Points join the track, detection, signalling and timetable. If their status is uncertain, restrictions can extend beyond the small piece of track involved. A control centre may need to adjust the service pattern because a useful route is unavailable. The effect depends on what alternative paths actually exist, not simply on the number of tracks visible.

Different places, different designs

Interlockings may be mechanical, relay-based or computer-based. Grade-separated junctions remove some crossing conflicts by putting tracks on different levels, but still require route control. A simple two-track metro and a branching suburban railway have very different junction problems. The conceptual diagram does not show operating rules for either.