Which railway signalling system uses colored lights to tell train drivers when they may proceed?

Colour light signalling uses illuminated colored aspects to tell train drivers whether they may proceed and, when necessary, how fast they should travel.

A typical system uses red for stop, yellow or amber for caution, and green for proceed. Railways may use additional aspects or combinations to communicate speed restrictions and the condition of several track sections ahead.

The signals work with track circuits, axle counters, interlockings, and control systems. These technologies help determine whether a section is occupied and prevent conflicting routes from being cleared at the same time.

Colour light signalling differs from semaphore signalling, which communicates with a mechanical arm or blade. Both systems can be integrated with modern automatic train protection, so a visible signal is only one part of the complete safety system.