Bi-directional railway signalling

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.

Before discussing the application of signals, it is useful to highlight some situations where signals are not required:

Signals exist primarily to pass instructions and information to drivers of passing trains. The driver interprets the signal's indication and acts accordingly. The most important indication is 'danger', which means 'stop'. Not every signal is equipped with a 'danger' aspect.

Signals are provided where required to protect items of infrastructure where conflict may arise, including:

A signal cannot display a 'proceed' aspect unless the infrastructure item(s) that it protects are in the correct position for the passage of a train or, in the case of flat junctions and gauntlet track, no other signal is cleared for a conflicting move. This is enforced by interlocking.

Running lines (as distinct from sidings) are divided into sections. Under normal circumstances, only one train may occupy any section at a time. A signal is provided at the start of every section, which may only display a 'proceed' aspect when the section ahead is completely empty, at least as far as the next signal.

The length of the sections, and hence the distance between signals, determines the railway's capacity. A railway with short sections can accommodate more traffic than one with long sections.

The provision of signals is dependent on the intended use of the layout and the movements that are expected to take place. It is not necessary to provide a signal for every conceivable movement that the layout physically permits. 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. The same applies during signal failure.

A main signal controls a train movement along a running line main line. A 'proceed' aspect on a main signal indicates that the line is clear at least as far as the next signal, giving the driver confidence to run at speed. Trains running long distances, especially passenger trains, will usually travel throughout under the authority of a succession of main signals.

A shunting signal controls low speed movements where provision of a main signal is not appropriate, such as moves into sidings. Clearance of a shunting signal does not necessarily imply that the line ahead is clear of vehicles. Shunting signals are often mounted at ground level and are smaller than main signals, reflecting their status.

A stop signal is one that is equipped to show a 'danger' aspect, which commands an approaching train to stop. Its function is to prevent conflict with other trains and to indicate that moveable infrastructure features are in the correct position. Depending on the manner in which they are used, and the nomenclature favoured by the railway administration concerned, stop signals may be further categorised as 'home signals' or 'starting signals', for example. Some stop signals are in the form of a fixed signal, generally a white board with a red solid circle. There is usually a panel underneath with instructions to the driver as to what circumstances he may pass the signal. Examples of such signals were used on lines signaled by the radio electronic token block system.

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Wikipedia