THE SIGNAL BOX

SIGNALS

SEMAPHORE SIGNALS

2. Stop and Distant signals

Stop signals

Stop signal (lower quadrant) Stop signal (somersault) Stop signal (upper quadrant)

A stop signal has a red arm with vertical white stripe towards the left-hand end. It shows a red light when on and a green light when off. Trains must not pass stop signals when on except under specially authorised conditions - this is one of the most contentious issues of modern-day railway operation.

Note - in certain circumstances, the Colour Light equivalent may show red, yellow and green indications.


Two-aspect Colour Light stop signal
Lower quadrant Somersault Upper quadrant Colour Light

At any signal box, there may be several stop signals serving each line. Most commonly there are two, with the first to be reached by a train (known as the home signal) protecting any points, level crossings etc., and the second (the starting signal) ahead of any points and guarding the entrance to the block section ahead. More complex layouts can have many stop signals per line, perhaps named Outer Home, Intermediate Home, Inner Home, Starting, Advanced Starting, Outer Advanced Starting) whilst in smaller circumstances just a single home signal may suffice.

Whichever of the signals allows entry into the block section ahead is also termed the section signal, but this is more of a technical term and is not normally used from an operating point of view.

The colour light equivalent can be in two forms. It will show red or green if there are no other stop signals ahead worked from the same signal box (and as long as it isn't also acting as a distant signal - explained later) but if there is another stop signal ahead it will show yellow until all such signals are cleared.

A small word about Rule 39 is appropriate here. This requires a signalman, if not in a position to clear all signals for a train (perhaps the section ahead is still occupied by the previous train) to bring the train nearly to a stand at each of his signals before allowing it to draw forward to his next stop signal.

Distant signals

Distant signal (lower quadrant) Distant signal (somersault) Distant signal (upper quadrant)

A distant signal has a yellow arm with a vee-notch at the left end of the arm. A black vee-stripe is painted towards the left end. Distant signals show a yellow light when on and a green light when off.

A distant signal is the first signal a driver sees when approaching a signal box, and when off it indicates that ALL stop signals controlled from the box ahead are off. If on, a driver must expect to stop at the first stop signal. By definition, therefore, a distant signal has to be located a full braking distance from that stop signal, taking into account gradients and maximum permitted speeds.


Two-aspect Colour Light distant signal
Lower quadrant Somersault Upper quadrant Colour Light

Taking account of the description of Rule 39 above, it can be seen that if a driver passes a distant signal at on, he will be prepared to find the stop signal ahead on, but if that signal is off (without his brought nearly to a stand) he can expect all stop signals ahead (worked by that box) to be off.