Color signals are a type of fixed wayside signal that communicate instructions and information to train crews through the display of colored lights. They are one of the most common signaling methods used throughout the railway industry today.
The primary method of communication is the color of the light displayed. Red, yellow, and green are the most common colors used, although some systems also use lunar white, blue, or purple for specialized indications.
Color signals are designed to provide train crews with clear, visible, and unambiguous information regarding movement authority, speed restrictions, route selection, and conditions ahead.
The colored lights are housed within a signal head that contains lenses and light sources. Modern systems typically use LED technology because of its reliability, brightness, and reduced maintenance requirements.
Different signal aspects are created by illuminating specific colors or combinations of colors. A single color may convey one instruction while a combination of colors can convey more complex operating information.
For example, a single green aspect may indicate that a train is authorized to proceed, while a combination of lights may provide information about speed restrictions or diverging routes ahead.
Some color signal systems use flashing lights to provide additional information. A flashing aspect may indicate a less restrictive condition or modify the meaning of the displayed color.
Color signals are specifically designed for excellent visibility during both daytime and nighttime operations. Their brightness and lens design allow train crews to recognize signal indications from considerable distances.
The operation of color signals is controlled by the railroad's signaling system. Signals may respond automatically to track conditions, track occupancy, and switch positions, or they may be controlled through interlockings and dispatching systems.
Every signal aspect has a specific meaning defined by the railroad's operating rules. Train crews must understand and correctly interpret every signal indication they encounter.
Because train safety depends upon proper signal interpretation, signal aspects and operating rules are a critical part of railroad training and qualification programs.