Color light signals are one of the most important communication tools used in railway operations. They provide train crews with critical information that governs how trains are operated safely and efficiently throughout a signaling territory.
The most basic information conveyed by a color signal is movement authority. A signal indication informs train crews whether they are authorized to proceed, must prepare to stop, or are required to stop immediately.
Green signal aspects generally indicate that a train is authorized to proceed. This usually means the block ahead is clear and movement may continue according to the applicable operating rules.
Red signal aspects generally require a train to stop before passing the signal. Movement beyond a stop indication is prohibited unless authorized under specific operating rules.
In Automatic Block Signaling territory, color signals help maintain safe separation between trains by indicating the occupancy status of the block ahead.
Yellow signal aspects often provide advance warning that a more restrictive signal lies ahead. This allows train crews to begin braking and adjust train handling before reaching the next signal.
Color signals can also communicate speed restrictions. Certain signal aspects require trains to operate at reduced speed when approaching turnouts, crossovers, interlockings, or other restricted locations.
Route information is another important function of color signals. At interlockings, signal aspects may indicate whether the train will continue on the main track or follow a diverging route through switches and crossovers.
Every signal aspect has a specific meaning defined by the railroad's operating rulebook. Safe train operation depends on the crew's ability to correctly interpret and comply with these signal indications.