Colour light signals

A signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train/engine drivers (engineers in the US). The driver interprets the signal's indication and acts accordingly. Typically, a signal might inform the driver of the speed at which the train may safely proceed or it may instruct the driver to stop.

Originally, signals displayed simple stop/proceed indications. As traffic density increased, this proved to be too limiting and refinements were added. One such refinement was the addition of distant signals on the approach to stop signals. The distant signal gave the driver warning that he was approaching a signal which might require a stop. This allowed for an overall increase in speed, since train drivers no longer had to drive at a speed within sighting distance of the stop signal.

Under timetable and train order operation, the signals did not directly convey orders to the train crew. Instead, they directed the crew to pick up orders, possibly stopping to do so if the order warranted it.

Signals are used to indicate one or more of the following:

Signals can be placed:

'Running lines' are usually continuously signalled. Each line of a double track railway is normally signalled in one direction only, with all signals facing the same direction on either line. Where 'bi-directional' signalling is installed, signals face in both directions on both tracks (sometimes known as 'reversible working' where lines are not normally used for bi-directional working). Signals are generally not provided for controlling movements within sidings or yard areas.

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; be 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 coloured aspects, the colour 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.

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Wikipedia