Application of railway signals

The application of railway signals on a rail layout is determined by various factors, principally the location of points of potential conflict, as well as the speed and frequency of trains and the movements they require to make.

The provision of signals is dependent on the intended use of the layout and the movements that are expected to take place.

If, during perturbed working, a movement has to be made in a direction for which no signal is provided, this can be done under special instruction.

Its function is to prevent conflict with other trains and to indicate that moveable infrastructure features are in the correct position.

The distant signal is installed at roughly full braking distance on approach to the stop signal to which it applies, taking into account the gradient, the permitted speed, and the braking performance of trains.

If the distant signal shows a 'clear' aspect, the train may maintain full speed.

An alternate form is a picture of a horizontal distant arm painted onto a white board.

Drivers are permitted to pass the signal at 'danger' under their own authority, in accordance with the rules, after first coming to a stand.

These are commonly used at terminal stations to permit two or more trains to enter a single platform.

The signal will display a restrictive aspect to make the driver reduce the train's speed.

Bidirectional signalling intended for regular use will generally allow traffic to flow at similarly high frequency in one or other direction.

Bidirectional signalling is more expensive to implement as it requires more equipment than uni-directional operation, so it is not always provided.

Before power-operated signalling became widespread, track layouts tended to be designed to avoid bidirectional arrangements as much as possible.

Signals may be installed to control the movement of freight trains through a facility for loading or unloading minerals, for example coal.