Railway Signal Aspects and Indications
Signals have aspects and indications. The aspect is the visual appearance of the signal; the indication is the meaning. In American practice the indications have conventional names, so that for instance “Medium Approach” means “Proceed at not exceeding medium speed prepared to stop at next signal”. Different railroads historically assigned different meanings to the same aspect, so it is common as a result of mergers to find that different divisions of a modern railroad may have different rules governing the interpretation of signal aspects.
It is important to understand that for signals that use colored aspects, the color of each individual light is subsumed in the overall pattern. In the United States, for example, it is common to see a “Clear” aspect consisting of a green light above a red light. The red light in this instance does not indicate “Stop”; it is simply a component of a larger aspect. Operating rules normally specify that when there is some imperfection in the display of an aspect (e.g., an extinguished lamp), the indication should be read as the most restrictive indication consistent with what is displayed.
Signals control motion past the point at which the signal stands and into the next section of track. They may also convey information about the state of the next signal to be encountered. Signals are sometimes said to “protect” the points/switches, section of track, etc. that they are ahead of. The term “ahead of” can be confusing, so official UK practice is to use the terms in rear of and in advance of. When a train is waiting at a signal it is “in rear of” that signal and the danger being protected by the signal is “in advance of” the train and signal.
A distinction must be made between absolute signals, which can display a “Stop” (or “Stop and Stay”) indication, and permissive signals, which display a “Stop & Proceed” aspect. Furthermore, a permissive signal may be marked as a Grade Signal where a train does not need to physically stop for a “Stop & Proceed” signal, but only decelerate to a speed slow enough to stop short of any obstructions. Interlocking (‘controlled’) signals are typically absolute, while automatic signals (i.e. those controlled through track occupancy alone, not by a signalman) are usually permissive.
Drivers need to be aware of which signals are automatic. In current British practice for example, automatic signals have a white rectangular plate with a black horizontal line across it. In US practice a permissive signal typically is indicated by the presence of a number plate. Some types of signal display separate permissive and absolute stop aspects.
Operating rules generally dictate that a dark signal must be interpreted as displaying its most restrictive aspect (generally “Stop” or “Stop and Proceed”).