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Section 2: Main Signals

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The 'position light' form of main signal, which was used extensively in parts of North America, underwent limited trials in Britain. This signal was capable of displaying three aspects, each consisting of four lights in a straight line that mimicked a semaphore arm. The angle of inclination determined whether the signal was displaying 'danger' [2.78], 'caution' [2.79] or 'clear' [2.80]. As well as showing the same aspects at night as it did by day, the position light signal had the advantage that it completely eliminated colour. A disadvantage was its large size, comparable to that of a semaphore signal, precluding its use in confined spaces such as tunnels. There was also a risk of misreading the indications at locations such as stations where a large number of white lights would be present for other purposes. One of these signals was installed at Willesden Green on the Metropolitan Railway in 1918, co-acting with a semaphore signal (see Section 7). A second example appeared on the L&SWR at Waterloo in 1920 but use of the position light as a main signal failed to catch on in Britain.

[2.78] Position Light Signal showing 'Danger'.
Area: Willesden Green, Met.R / London Waterloo, L&SWR   Usage: Low   Status: Historical
[2.79] Position Light Signal showing 'Caution'.
Area: Willesden Green, Met.R / London Waterloo, L&SWR   Usage: Low   Status: Historical
[2.80] Position Light Signal showing 'Clear'.
Area: Willesden Green, Met.R / London Waterloo, L&SWR   Usage: Low   Status: Historical

The GWR's Ealing & Shepherd's Bush Railway opened in 1917. Three-position semaphore signals were provided in 1920. Uniquely, those that worked automatically were distinguished by having a pointed arm with a matching white 'vee' [2.81 - 2.83]. Note that a similar style of signal arm had been used for level crossing signals on the Dornoch Light Railway (see [16.3 & 16.4]).

[2.81] Automatic Three-position Semaphore Signal showing 'Danger'.
Area: Ealing & Shepherd's Bush Railway   Usage: Low   Status: Historical
[2.82] Automatic Three-position Semaphore Signal showing 'Caution'.
Area: Ealing & Shepherd's Bush Railway   Usage: Low   Status: Historical
[2.83] Automatic Three-position Semaphore Signal showing 'Clear'.
Area: Ealing & Shepherd's Bush Railway   Usage: Low   Status: Historical

Gradually, all the other railway companies followed the example of the Great Central Railway in painting the arms of their distant signals yellow and fitting yellow lenses in place of the red (see [2.76 & 2.77]). On the Great Northern Railway, the yellow distant signals had vertical black bands [2.84 & 2.85].

[2.84] Semaphore Distant Signal with Yellow Arm ('on').
Area: GNR   Usage: High   Status: Historical
[2.85] Semaphore Distant Signal with Yellow Arm ('off').
Area: GNR   Usage: High   Status: Historical

Other companies that used somersault signals put black chevrons on their distant signals [2.86].

[2.86] 'Somersault' Distant Signal with Yellow Arm and Black Chevron ('off').
Area: Various   Usage: High   Status: Historical

The earliest yellow distant signals on the London & South Western Railway had white chevrons on the front of the arms [2.87 & 2.88].

[2.87] Semaphore Distant Signal with Yellow Arm ('on').
Area: L&SWR   Usage: Unknown   Status: Historical
[2.88] Semaphore Distant Signal with Yellow Arm ('off').
Area: L&SWR   Usage: Unknown   Status: Historical

As an alternative to using banner type main signals in restricted clearance areas (see [2.70 & 2.71]), the Great Western Railway provided mechanical disc type main signals at Worcester and Gloucester [2.89 & 2.90]. Nicknamed 'banjos', these signals were enlarged versions of disc shunting signals (see [3.39 & 3.41]).

[2.89] Disc Stop Signal ('on').
Area: GWR   Usage: Low   Status: Obsolescent
[2.90] Disc Stop Signal ('off'). Click Here for Photo
Area: GWR   Usage: Low   Status: Obsolescent

Colour light signals employ no moving parts, display the same aspects by day and by night, and are sufficiently compact as to be accommodated even where space is limited. The first use of colour light signals in Britain was on the Liverpool Overhead Railway in 1920. These were two-aspect signals. Colour light signals capable of displaying three aspects first appeared on a mainline railway in 1923, on the LNER line between Marylebone and Neasden. This scheme was planned by the Great Central Railway in 1922, A.F. Bound being the engineer responsible. The three aspects corresponded to the night-time indications of the upper quadrant three-position semaphore signals (see [2.9, 2.73 & 2.74]). A red aspect means 'danger' [2.91], a yellow aspect means 'caution' [2.92], and a green aspect means 'clear' [2.93].

[2.91] Red Aspect.
Area: All Areas   Usage: High   Status: Current
[2.92] Yellow Aspect.
Area: All Areas   Usage: High   Status: Current
[2.93] Green Aspect.
Area: All Areas   Usage: High   Status: Current

In 1922, the Institution of Railway Signal Engineers set up a committee to consider the question of three-position signalling and make recommendations as to the most suitable signal aspects. Its report, published in 1924, went further and recommended the introduction of a fourth aspect in colour light signals, for use in congested areas where a better headway was required. The new aspect was given the meaning "warning - be prepared to find the next signal at 'caution'". The recommended aspect was two yellow lights, one placed vertically above the other [2.94], although one yellow combined with one green had been considered. The new double yellow aspect was first used between Holborn Viaduct and Elephant & Castle on the Southern Railway, in 1926.

[2.94] Double Yellow Aspect.
Area: All Areas   Usage: High   Status: Current

Another recommendation of the IRSE's Three-Position Signalling Committee report was that, in future, all three-aspect signals should be of the colour light type. The recommendation to discontinue the installation of three-position semaphore signals left the way open for the introduction of two-position upper quadrant semaphores in place of lower quadrant ones. Their arms are raised through 45° for the 'off' position [2.95 & 2.96] and so are less susceptible to 'wrong side' failures than lower quadrant signals.

[2.95] Upper Quadrant Semaphore Stop Signal ('off').
Area: All Areas   Usage: High   Status: Current
[2.96] Upper Quadrant Semaphore Distant Signal ('off').
Area: All Areas   Usage: High   Status: Current