This electrically powered signal unit uses high-intensity lamps behind red, yellow, and green lenses to display aspects to train crews. Unlike older mechanical signals, they have no moving external parts and rely purely on the combination of lights to dictate speed and route. They are the standard method of controlling train movements in mainline territory.
Heard on the job
You have a clear color light signal at the east end of the siding, so bring her up to track speed.
Related terms
Block signal
A fixed trackside light or semaphore that tells an engineer whether the track segment ahead is clear.
Aspect
The visual appearance of a trackside signal as seen by an approaching locomotive crew.
Home signal
An absolute signal that governs a train's entry into a specific interlocking or block.