Fixed wayside signals communicate information to train crews by displaying signal aspects. The method used to display these aspects depends on the type of signal installed along the railroad.
Color light signals are the most common signal type used today. These signals use colored lenses and light sources, typically LEDs, to create different signal indications.
Different aspects are created by illuminating specific combinations of red, yellow, green, lunar white, or other colored lights. The arrangement and color of the illuminated lights determine the meaning of the signal.
Some color light signals use flashing indications to provide additional information. For example, a flashing yellow signal may convey a different instruction than a steady yellow signal.
Multiple signal heads may be mounted on a single mast. These additional heads allow the signal system to display route information, speed commands, and more complex operating instructions.
Semaphore signals display aspects using a movable arm mounted on a signal mast. The position of the arm determines the signal indication.
A horizontal semaphore arm typically indicates stop, while downward arm positions indicate various proceed or caution aspects depending on the railroad's operating rules.
Position light signals use rows of white or amber lights arranged in specific patterns. The pattern created by the illuminated lights determines the aspect being displayed.
Common position light aspects include a horizontal row of lights for stop, a vertical row for proceed, and diagonal rows for approach indications.
Searchlight signals use a single lens and an internal mechanism that changes the color projected through the lens. The signal can display red, yellow, or green indications using the same signal head.
Regardless of signal type, all signal aspects are carefully standardized and governed by railroad operating rules to ensure consistent and safe communication with train crews.
Clear and unambiguous signal displays are essential for maintaining safe train separation, efficient traffic flow, and compliance with railroad operating regulations.