colour light signal
An electric railway signal with colour aspects, showing either red or green in a 2-aspect signal, with the addition of a yellow or double yellow aspect (meaning caution) in 3- and 4-aspect signals.
colour light signal: electric railway signal using colour and aspect rules
A colour light signal is an electric railway signal that conveys movement authorities and warnings to train drivers using coloured light aspects rather than mechanical semaphore arms. The signal displays distinct colours, each carrying a specific instruction about line conditions and permitted speed ahead. The signal head contains electric lamps, usually incandescent or LED, behind coloured filters or lenses, and the circuit is controlled by track circuits, interlocking equipment, or a signal box operator.
A 2-aspect signal shows red (stop) or green (proceed). The 3-aspect signal adds yellow (caution, prepare to stop), typically meaning the next signal ahead is at stop. A 4-aspect signal introduces double yellow (caution, next signal is at caution), giving drivers earlier warning of signal changes ahead. Some railways use 5-aspect or more complex systems, but 2-, 3-, and 4-aspect are the most common configurations. The choice depends on line speed, traffic density, and braking performance of rolling stock.
Colour light signals replaced mechanical arms because they are more reliable in poor visibility, require less maintenance, and allow remote control from centralised signal boxes. A single signal post may carry multiple signal heads to show different routes, with each head controlling a particular line or direction. The arrangement is called a signal gantry or bracket signal when mounted above the track or on a cantilevered arm.
Faults and limitations
Colour blindness in drivers is a recognised hazard, which is why many railways supplement colour aspects with position-light signals or additional audible cab signalling. Lamp failure must be detected by the interlocking circuit; a failed green lamp is more dangerous than a failed red lamp because it could show unintended green. Signal wiring must be checked regularly for corrosion and loose connections. In severe weather, snow or ice can obscure the lens, and some signals include heaters or wipers to prevent obscuration.
Colour light signals became standard on main lines from the 1920s onward. Their logic is defined in railway regulations specific to each country or region. UK railway colour light signals are governed by Railway Group Standards; German, French, and other national railways have their own aspect definitions and placement rules. The term itself reflects the distinction from older semaphore signals and modern digital signalling systems that use radio cab signalling or moving block principles.