|1. Running Signals.
These are the most well-known, and easily recognised even by people with no interest in railways. They fall into two categories, the Stop signal and the Caution signal.
Arms with a red face and a white vertical stripe on the front whilst the reverse is white with a black stripe are Stop signals. Stop signals are mainly used before a signalbox/station as home and frequently outer home signals, beyond a signalbox/station as starting and frequently advanced starting signals. Yellow "fish-tailed" signals with a black ">" stripe on the front and white with a black "< " stripe on the reverse are Caution signals.
Left: Stop (starting) signal at Newhaven Marine.
photographs by Glen Woods
Right: Caution (distant) signals at Lewes.
The Caution signal (then known as Auxiliary) was first used in 1848 by the L&SWR and was the same shape and colour as the Stop signal. In 1872 the LB&SCR introduced the fish-tail arm, though they had the same vertical white stripe as the Stop arm and also showed a red light after dark so to help differentiate between a Stop and a Caution signal a device called a Coligny-Welch lamp was used, but only on the LB&SCR, the L&SWR and the Furness Railway. This lamp exhibited a small white chevron to the right of the signal lamp thereby making night-time identification somewhat easier. The modern yellow signal arm, as shown here, was only introduced from the late 1920s.
When a signal's arm is horizontal it is said to be "on" and when it is either raised or lowered, it is said to be "off". Their meanings are quite simple, when the Stop signal is "on" the train must stop and when it is "off" the section is clear all the way to the next Stop signal and the train may proceed, whilst if a Caution (more commonly known as a Distant) signal is "on" it tells the driver he must be prepared to stop at a Stop signal ahead whilst when it is "off" it tells him the next Stop signal (or a series of Stop signals) is also "off". It therefore follows that a Driver may pass a Distant signal that is "on" but may never pass a Stop signal that is "on" unless specifically authorised to do so by the Signalman, or other person in charge of the line.
Sometimes there will be two arms on a post, one home arm and one distant arm. Obviously, if the home arm is not "off", then the distant arm cannot be.
Right: Stop and caution arms on the same posts at Selsdon, one a high post, the other a low one. This would have been for sighting purposes
photograph by kind permission of Allan Elliott.
For night use the signals will display a green light when "off", a red light for a Stop signal which is "on" or a yellow light for a Distant signal which is "on". A Fixed Distant can, of course, only ever have a yellow light.
Very early signals rotated, but quite soon most signals were lowered to the "off" position and are called lower-quadrant but there were some accidents caused by broken signal wires allowing the arm to fall when it shouldn't which led many railways to adopt the raised arm, or upper-quadrant signal. (Note there are different arrangements between the two designs of signal's spectacle plates that show a coloured light after dark). Those railways that stayed with the lower-quadrant arm introduced a weight to counter the weight of the signal arm so that if the wire were to break the signal would return to danger. Subsequently the counter-weight was introduced to upper-quadrant signals as well.
The bracket signal can carry one, two, or more, raised posts, called "dolls" arranged according to the route they control. Where just one doll is carried it is normally to site the signal conveniently for the driver where there is a problem using a single post.
Left: Bracket stop signal at the east end of Reigate station.
photograph by Keith Harwood
Where more than one doll is provided the "main" route will always be controlled by the signal on the highest doll so if a bracket signal controls a junction with a branch line or siding leading off to the left, then the main route will be controlled by the higher arm, usually mounted on a doll directly above the supporting post below the bracket, whilst the line branching off will be controlled by the signal on a shorter doll, off-set to the left. If both dolls are the same height, then they will be equidistant from the supporting post and each route has the same importance. A bigger development of the bracket signal is the gantry signal, spanning several tracks, where the route controlled is identified by reading the signal arms from left to right, the importance of the route being reflected by the height of the signal arm. Where a single post carries two or more Stop arms, then as you read the arms down the post from top to bottom they apply to the routes from left to right.
A further form of multi-arm signal is the co-acting signal where, through sighting difficulties, arms are duplicated at the top and bottom of a signal post. This can be particularly useful to the driver of a fast train proceeding non-stop through a station with a footbridge obscuring the sighting of the starting signal at the end of the platform, which otherwise would require him to slow and be prepared to come to a stand. A driver waiting for the right away would, of course, look at the "normally situated" lower arm.
In the early days of signals being provided there were various designs tried, including roatating, somersault, slotted posts (where the arm disappeared completely) but it soon settled down so that virtually all the signal arms in use were of the lower quadrant (i.e. dropped from the horizontal) variety, though there were experiments with three-position semaphore arms. In 1926 it was recommended that three-position semaphore arms be replaced and that single multiple signals should be represented by colour light signalling. Following this the lower quadrant arm was largely replaced by the upper quadrant arm on the Southern, LMS and LNER, although examples of lower quadrant arms remained in place many years after the nationalisation of the railways in 1948. The first use of upper quadrant arms on the Southern Railway is thought to have been at Liphook in March 1928 when "As an experiment, the existing standard semaphore arms of the down distant, home and starting (with lower duplicate arm) and up distant, home and starting (with lower duplicate arm) and advanced starting signals will be substituted by 2-position upper-quadrant arms".
2. Shunting Signals.
The function of a shunt signal (or "dummy") is simple, when it is "off" it permits the driver to pass, but unlike a running signal, only for as far as he can see the line to be clear. It does not necessarily give him permission to proceed as far as the next signal and he must be prepared to stop at any time after passing the signal.
Left: Shunt signal at Newhaven Marine.
photograph by Glen Woods
The type of shunt signal illustrated here was to be found all over the Southern Railway and BR(S), was made by Westinghouse Brake and Signal Co. and was introduced sometime after the grouping
If the signal is black with a yellow stripe, then it may be passed when "on" providing the movement is not in the direction controlled by the signal, which avoids the need for lots of shunt signals in sidings which can be safely operated by a shunter. Such a signal could be used to control the exit from sidings, so having first obtained the Signalman's permission, a shunting engine can then pass the signal in other directions for as long as is needed, or until the Signalman withdraws the authority to do so.
The points and signals are operated from lever frames. Most of these levers are in the signalboxes but some can be outside, in a ground frame. Ground frames are normally used for access/exit from sidings that are too far away to be contolled by a Signalbox, or where there is insufficient room on the Signalbox's main frame. These ground frames will all be locked to avoid unauthorized use, those in a section by a key and those close to a Signalbox by a lever within the 'box. Red levers control the Stop signals, yellow ones the Distant signals, black levers operate the points and blue ones lock the points. Brown and blue levers are locked releases and would, for example, be used to control a ground frame. Brown and white ones are used to close a box with points locked and signals "off" permitting movement of trains on one line only when the signalbox is un-manned ("switched out"). White levers are spare levers and green levers would be used for some non-specific use such as operating a gong in a yard to warn of an impending movement.
3. Information Signals.
An important form of information signal is the Banner repeater signal. This is an illuminated round white signal with a black horizontal stripe - oblong for a Home signal or fishtailed for a distant - which works by the arm rotating to mimic the movement of a semaphore arm. Modern ones use an electronic display.
Right: Modern banner repeater signals at London Bridge
photograph by Erinrail
They are used to give advance warning to a driver of the condition of a signal ahead that may be obscured from his view until he is almost on top of it. A classic example is at a station that either has a curved platform so that he cannot see the starting signal until well on the way through the station, or has a platform canopy or footbridge obscuring his view (though in the latter example a co-acting signal might well be used unless there are sighting difficulties). These may, of course, be passed when "on" as they only advise the driver what is ahead of him, not the situation ruling where the banner signal is sited. A route indicator can be attached to a signal post and will advise a driver which road is signalled, thereby reducing the number of arms required. When the signal is "on" no indication is shown but when the signal is "off" then a number or a letter is displayed appropriate to the route selected.
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This page was last updated 2 January 2011