The aspects of a color signal are the visual indications displayed through various combinations of colored lights. Each aspect has a specific meaning and requires a specific response from the train crew.
Signal aspects are created by illuminating one or more colored lights within the signal head. The colors displayed, their arrangement, and whether they are flashing or steady determine the meaning of the signal.
The most restrictive aspect is typically a red signal. A red aspect requires the train to stop before passing the signal and remain stopped until proper authority is received.
A green signal aspect generally indicates that the block ahead is clear and the train may proceed at the authorized speed in accordance with operating rules.
Yellow aspects are commonly used as approach indications. These aspects warn train crews that a more restrictive signal lies ahead and that they should prepare to stop if necessary.
Flashing yellow aspects may indicate a less restrictive condition than a steady yellow while still warning crews that conditions ahead require attention. They are often associated with diverging routes or speed restrictions.
Restricting aspects authorize movement at restricted speed. Examples may include red over yellow, yellow over red, flashing red, or lunar white, depending upon the railroad's operating rules.
When operating under a restricting aspect, train crews must proceed at restricted speed and be prepared to stop short of any obstruction, occupied track, broken rail, or improperly lined switch.
Diverging route aspects are commonly displayed at interlockings and control points. These aspects inform train crews that the train will travel through a turnout or crossover onto another route.
Examples of diverging route aspects may include combinations such as yellow over green, green over yellow, or yellow over yellow depending upon the railroad's signaling system and operating rules.
Some aspects authorize movement while also imposing speed restrictions. Examples include green over yellow or flashing green aspects that require trains to proceed but at a specified speed.
Every railroad publishes detailed signal charts and operating rules that define the exact meaning of every signal aspect. Proper interpretation of these aspects is critical to safe railroad operations.