In railway signaling, indications are the meanings or instructions conveyed to the train crew by the aspect displayed on a signal. While the aspect is what the crew sees, the indication is what the crew must do in response to that aspect.
A simple way to understand the relationship is to think of the aspect as the language of the signal and the indication as the message being communicated. The aspect provides the visual information, while the indication provides the operating instruction.
Every signal aspect has a specific indication defined by the railroad's operating rules. Train crews must know these rules thoroughly in order to operate safely and comply with railroad regulations.
Signal indications often provide instructions related to train speed. Certain indications authorize maximum speed, while others require reduced, medium, slow, or restricted speed operation.
Indications also communicate movement authority. They tell crews whether they are permitted to proceed beyond the signal, must prepare to stop, or are required to stop immediately.
Some indications prepare crews for conditions ahead. An approach indication may require a train crew to proceed while preparing to stop at the next signal if necessary.
Route information is another important function of signal indications. At interlockings, signals may indicate whether a train will remain on the main track or take a diverging route through switches and crossovers.
Signal indications can also provide information about block occupancy. More restrictive indications often warn crews that another train or obstruction may exist ahead.
Examples of aspect and indication relationships include a steady green aspect indicating "Proceed," a steady red aspect indicating "Stop," and a steady yellow aspect indicating "Approach."
Modern signaling systems often include a wide variety of aspects and indications to improve safety, increase capacity, and manage train movements more efficiently.