UK railway signalling
Their jobs were many and varied, but one of their key roles was the giving of hand signals to inform engine drivers as to the state of the line ahead.
Both types are fail-safe in the event of breakage of the operating pull-wire but lower-quadrant signals require a heavy counterweight with push-pull rod between counterweight and arm linkage (generally assisted by the "spectacle" that carries the coloured lenses for use at night) to do that, while upper-quadrant signals fall back to "danger" under the weight of the arm.
The stop signal consists of a red, square-ended arm, with a vertical white stripe typically 9-12 inches (230–300 mm) from the end, and advises the driver whether the line immediately ahead is clear or not.
On many branch lines and short block sections, a distant signal was often fixed at 'Caution', standalone or mounted below a Stop semaphore, and so exhibited only a yellow light at night.
Current British practice mandates that semaphore signals, both upper and lower quadrant types, are inclined at an angle of 45 degrees from horizontal to display an "off" indication.
[citation needed] The design considerations determining the familiar arrangement of roadway 'traffic signals', with red at the top, do not apply to the railway.
There are standard arrangements of the lights, however unusual variations, such as horizontal mounting is allowed when demanded by local conditions or geography, such as in tunnels, areas of limited clearance, or the presence of bridges over the railway.
[9] The concept had a renaissance in the 2000s with the advent of LEDs which allow the same aperture to be used to display multiple colours, while eliminating moving parts that could fail on searchlight signals.
After the track circuit has been occupied for the specified period of time, the signal is allowed to "step-up" to the highest available aspect and display the junction indicator where applicable.
In colour light power box operated areas, the "home" signal where "warning" arrangements are in force has a time release similar to approach control from red but the control is more stringent – the signal only clears when the speed of the train is detected to be less than 10 mph and only clearance to single yellow is allowed – this is called delayed yellow operation, and is often found at the approach to large stations where two trains may use one platform.
Today, all three look broadly the same; they are shaped like a normal semaphore stop signal, though only about two-thirds of the size, and are painted red with a white horizontal band running centrally along them.
When lit, with the main aspect showing red, they instruct the driver to proceed but be prepared to stop short of any obstruction.
Position light signals allow a train to move into a section under caution, the line ahead may be occupied so the driver must drive at a speed that enables them to stop short of any obstruction.
They are usually found at the exits of marshaling yards and sidings, and can be passed at danger for a movement in the direction for which the signal cannot be cleared (e.g. into a headshunt rather than onto the main line).
This arrangement removes the requirement for the signal to be cleared every time a shunt is to take place within the sidings without fouling the main running lines.
A small-arm semaphore shows "clear" in the same way as a full-size stop signal, while a disc rotates through 45 degrees or so when pulled off so that the red band is angled.
These consist of a black background, mounted under a single stop signal, on which is superimposed a white letter(s), number(s) or combination of the two, to make a code indicating the route to be taken.
In semaphore areas, route indicators may be mechanical, with boards that slide into view to display the code; or electric "theatre-type", with a light projected through a suitably-printed screen.
Mounted above a colour-light signal, they consist of a row of white lights (originally a single, long, u-shaped fluorescent tube in an open-fronted case), nowadays five but traditionally three, angled to the left or right depending on the direction of the divergence.
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.
[25] The original version, consisting of a black circle with a yellow outer ring and numbers, illuminated at night by a floodlight.
When it is necessary to carry out planned maintenance or repair work on a line, a temporary speed restriction (TSR) may be installed at a particular location.
[28] Train crew are notified of this in the Network Rail issued Weekly Operating Notice, which they are required to read before they book on duty.
Network Rail will fax information about the ESR to train crew booking-on points, where it must be displayed in the red-coloured 'Late notice case'.
The authority will allow the driver to pass the signal and proceed at a speed slow enough that they may stop short of any obstruction (in common with other degraded modes of operation) The term may be abbreviated to "PoSA".
[41] From 1914, a small number of British installations, notably, the Great Western and the South Eastern and Chatham, used motor-operated three-position semaphore signals of North American origin.
On repeater signals, the marker light was offset to the left-hand side of the post to indicate that the 'stop and proceed' rule applied.
During heavy fog, fogsignalmen would be stationed at distant signals with a lantern and detonators – small explosive charges that could be strapped to the rail to be exploded by the wheels of a train.
The fogsignalman's duty was to repeat the indication of the signal using their lantern; the semaphore arm was usually obscured by fog and hence invisible to the driver of a moving train.
[44] On parts of the Great Western Mainline and the Chilterns line the trial systems of Automatic Train Protection (ATP) are still in use.
