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 (ABS) territory, color signals help maintain safe separation between trains by indicating the occupancy status of the block ahead. A restrictive signal warns crews that another train may be occupying the track 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.
Some advanced signaling systems provide information about multiple blocks ahead. This allows train crews to anticipate future operating conditions and make smoother train handling decisions.
Restricting aspects authorize movement at restricted speed while requiring train crews to remain prepared to stop short of obstructions, occupied tracks, broken rails, or improperly aligned switches.
Color signals also serve as warning devices. Approach indications provide advance notice of conditions ahead and allow crews to safely adjust speed and braking performance.
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.