The visual indications displayed by fixed wayside signals are known as signal aspects. These aspects communicate specific instructions and information to train crews regarding train movement, speed, routing, and track conditions ahead.
Signal aspects may be displayed using colored lights, semaphore arm positions, or patterns of lights depending on the signal type used by the railroad.
One of the most important signal aspects is the stop aspect. In a color light system, this is usually displayed as a red light. In semaphore signaling, the arm is typically horizontal. This aspect requires a train to stop before passing the signal.
Proceed aspects authorize train movement. In color light systems, a green signal commonly indicates that the block ahead is clear and the train may proceed at the authorized speed.
Semaphore signals often display proceed indications by lowering the signal arm to a downward angle. Position light signals typically use a vertical arrangement of lights to indicate a proceed aspect.
Approach aspects provide advance warning that a more restrictive signal lies ahead. A yellow signal is commonly used for this purpose and informs train crews to be prepared to stop at the next signal if necessary.
Some approach aspects may also require speed reductions depending on the operating rules of the railroad.
Restricting aspects authorize train movement under special conditions and typically require operation at restricted speed. Train crews must be prepared to stop short of obstructions, occupied tracks, improperly lined switches, or other hazards.
At interlockings, signals often display diverging route aspects. These aspects inform train crews that the train will follow a route through switches onto another track or through a crossover.
Diverging route aspects are commonly displayed using combinations of colored lights arranged in specific patterns. These aspects may also include speed restrictions associated with the selected route.
Signal aspects vary between railroads and operating territories. Train crews must be thoroughly familiar with the signal rules, charts, and operating instructions applicable to their territory.
Correct interpretation of signal aspects is essential for maintaining safe train separation, preventing collisions, and ensuring efficient railroad operations.