How do railway signals combine train detection with route locking?

A train needs distance to stop, so railway signaling does more than choose a color at a junction. In a fixed-block arrangement, the railway is divided into defined sections. Train detection provides information about whether a section can be treated as clear. Signals and other movement authorities then constrain where a train may proceed.

One detection method is a track circuit. With no train present, electrical current through the rails can energize a receiving relay. Conducting wheelsets on an occupying train connect the rails and divert current from that relay. A loss of the expected clear indication is treated conservatively rather than interpreted as proof that the track is empty. Axle counters and other detection technologies work differently.

An interlocking combines this information with route conditions. It checks that required points are in their detected positions and locked, and prevents incompatible routes from being authorized together. An apparently empty section is therefore only one condition; an unsecured switch or conflicting reserved route can still prevent permission.

In the pictured example, permission requires all three conditions together: a clear detected block, proved-and-locked points, and no conflicting reservation. Occupied or unknown detection withholds permission. This combines evidence about where a train may be with control over where its route will lead. An apparently empty stretch alone is insufficient: the railway must also prevent a different authorized movement from entering an incompatible route.