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Concept
Railway signal
Summary
A railway signal is a visual display device that conveys instructions or provides warning of instructions regarding the driver’s authority to proceed. 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.
Application of railway signals
Originally, signals displayed simple stop or 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 they were 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:
That the line ahead is clear (free of any obstruction) or blocked
That the driver has permission to proceed
That points (also called switch or turnout in the US) are set correctly
Which way points are set
The speed the train may travel
The state of the next signal
That the train orders are to be picked up by the crew
Signals can be placed:
At the start of a section of track
On the approach to a movable item of infrastructure, such as points or switches or a swingbridge
In advance of other signals
On the approach to a level crossing
At a switch or turnout
Ahead of platforms or other places that trains are likely to be stopped
At train order stations
'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.
Official source
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