Color light signals are the most commonly used fixed wayside signals in modern railway signaling systems. They communicate information to train crews using colored lights that are displayed individually or in specific combinations.
These signals have become the standard throughout much of the world because they are reliable, highly visible, and easy for train crews to interpret during both daytime and nighttime operations.
A typical color light signal contains multiple lenses mounted within a signal head. Behind each lens is a light source, traditionally an incandescent lamp and increasingly an LED module.
The primary colors used in color light signaling are red, yellow, and green. These colors provide the basic instructions that govern train movement throughout a signaling territory.
Red generally indicates stop. A train receiving a red signal must stop before passing the signal unless authorized to proceed under specific operating rules.
Green generally indicates proceed. This aspect authorizes train movement at the speed permitted by the signal indication and operating rules.
Yellow commonly indicates approach or caution. This warns train crews that a more restrictive signal lies ahead and that speed reductions or stopping preparations may be required.
More complex information can be conveyed through combinations of lights, flashing indications, multiple signal heads, and additional colors such as lunar white, blue, or purple where permitted by operating rules.
Color light signals are frequently used to communicate route information, speed restrictions, block occupancy conditions, and movement authority through interlockings and control points.
Their flexibility allows railroads to display a wide variety of signal aspects while maintaining a consistent visual format that train crews can quickly recognize and interpret.
Color light signals are used extensively in Automatic Block Signaling (ABS), Traffic Control Systems (TCS), Centralized Traffic Control (CTC), and interlocking systems.
Because of their reliability, visibility, and versatility, color light signals remain a cornerstone of modern railroad signaling and train control.