British Railway Signals - Junction Signals

Junction Signals

• Left route

• First left route

• Route indicator

British signalling is unusual in that when a train approaches a junction and is taking a diverging route, the preceding signal will indicate the diverging route to the driver. A fixed lineside sign shows the speed that applies through the divergence. Most signalling systems around the world adopt the speed signalling philosophy, where the safe route speed is shown with no indication of the divergence at a junction.

Semaphore signals at junctions normally have a separate arm for each route, usually reading left-to-right, although in confined or hard-to-see locations the arms may be mounted above one another, reading left to right from the top downwards. Such arrangements are known as splitting signals. It is normal today to only find splitting stop signals, but traditionally splitting distants would be employed too; a series of side-by-side distant signals to give the driver advance warning of the route to be taken,so he could reduce his train's speed if necessary. Today, a single distant signal is generally used which displays caution if speed reduction is necessary. The height of each individual post of splitting signals (known as dolls) generally gives some information about each route; in some cases, the tallest doll applies to the highest-speed route; in others, the tallest doll applies to the most important route, be it slow or fast.

It is unusual to find a splitting signal with more than four dolls; if more route indications are required, a single stop arm with a route indicator - mechanical or electric - mounted underneath, is used. The route indicator may show an abbreviated version of the train's destination (usually at station exits), or a number (usually at station entrances to denote the platform into which the train is routed).

Colour-light signals which display whether the route is diverging from the route ahead will have a Route Indicator — colloquially called a feather— of white lights (originally a single, long, u-shaped fluorescent tube in an open-fronted case) attached to the main signal. If the feather is lit, then the train will be diverging from the main route at the next junction. A feather may point to the left or to the right, as appropriate. If individual light bulbs are used, each feather usually has five, although a few older examples installed by the Southern Railway only have three. A signal will have more than one feather where there is more than one diverging route ahead. The lit feather indicates which diverging line the train will be directed to. In the example shown, there are two routes diverging to the left of the main line, and the train will take the first one of the two.

In areas where speeds are lower and there are a number of routes which can be taken, alphanumeric (also called theatre-style) route indicators are used to display a number or a letter (e.g. a platform number or line designation) to denote the route the train is to take. They may be located above or beside the relevant signal. When a route is set and the signal is cleared, the relevant letter or number is shown. On shunting signals, where speeds are much lower, a miniature version of the alphanumeric route indicator is used.

When a route is set at a junction that involves the train taking a diverging route that must be passed at less than the mainline speed, a system known as approach release is used. There are a number of different types of approach release that are used on British railways but the most often used is approach release from red. This system has the signal before the diverging junction held at red until the train approaches it, whereupon it changes to a less restrictive aspect with the appropriate direction feather of five white lights. This is required so that the signals approaching show the correct caution aspects, slowing the train down for the junction. While the junction signal is held at red, the preceding signal will be displaying caution (yellow), and the one before that will display preliminary caution (double yellow) if it is a 4-aspect signal. This system allows for a gradual decrease in speed until a safe speed is reached for the train to move through the junction.

Another common system is approach release from yellow with flashing aspects in rear. It is essentially similar to approach release from red, except that the junction signal is released from yellow and the signals in rear will flash to warn the driver that the train will be taking a diverging route ahead. Where the turnout speed is the same as the mainline speed, approach release is not necessary.

On modern high speed routes, such as the East and West Coast Main Lines, some turnouts at major junctions are designed to operate at maximum or near maximum linespeeds to keep the average speed of the journey as high as possible and reduce journey times as well as unnecessary wear on the train wheels, brakes and the track. Movable frog switches are occasionally employed to allow high speed running through the junction.