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Section 2; pages:
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2,
3,
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5,
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Section 2: Main Signals
(Page 5 of 7)
The North American 'position light' type of main signal was tried out in Britain. This signal was capable of displaying three aspects, each consisting of four lights in a straight line that mimicked a semaphore arm. Its angle of inclination determined whether the signal was displaying 'danger' [2.79], 'caution' [2.80] or 'clear' [2.81]. The same aspects were shown by day or by night. One of these signals was installed at Edgware Road 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.
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[2.79] Position Light Signal showing 'Danger'.
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[2.80] Position Light Signal showing 'Caution'.
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[2.81] Position Light Signal showing 'Clear'.
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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' [2.82 - 2.84]. 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]).
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[2.82] Automatic Three-position Semaphore Signal showing 'Danger'.
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[2.83] Automatic Three-position Semaphore Signal showing 'Caution'.
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[2.84] Automatic Three-position Semaphore Signal showing 'Clear'.
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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 [2.77 & 2.78]). On the Great Northern Railway, the yellow distant signals had vertical black bands [2.85 & 2.86].
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[2.85] Semaphore Distant Signal with Yellow Arm ('on') (GNR).
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[2.86] Semaphore Distant Signal with Yellow Arm ('off') (GNR).
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Other companies using somersault signals put black chevrons on their distant signals [2.87].
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[2.87] 'Somersault' Distant Signal with Yellow Arm and Black Chevron ('off').
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The earliest yellow distant signals on the London & South Western Railway had white chevrons on the front of the arms [2.88 & 2.89].
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[2.88] Semaphore Distant Signal with Yellow Arm ('on') (L&SWR).
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[2.89] Semaphore Distant Signal with Yellow Arm ('off') (L&SWR).
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As an alternative to using banner type main signals in restricted clearance areas (see [2.72 & 2.73]), the Great Western Railway provided mechanical disc type main signals at Worcester and Gloucester [2.90 & 2.91]. Nicknamed 'banjos', these signals were bigger versions of disc shunting signals (see [3.37 & 3.39]).
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[2.90] Disc Stop Signal ('on') (GWR).
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[2.91] Disc Stop Signal ('off') (GWR).
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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, 2.74 & 2.75]). A red aspect means 'danger' [2.92], a yellow aspect means 'caution' [2.93], and a green aspect means 'clear' [2.94].
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[2.92] Red Aspect.
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[2.93] Yellow Aspect.
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[2.94] Green Aspect.
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The report of a committee set up by the IRSE in 1922 and published in 1924 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 would have 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 [2.95], although one yellow combined with one green had been considered. The new double yellow aspect was first used on the Southern Railway, in 1926.
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[2.95] Double Yellow Aspect.
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Another recommendation of the 1924 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 [2.96 & 2.97] and so are less prone to risk of 'wrong side' failure than lower quadrant signals.
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[2.96] Upper Quadrant Semaphore Stop Signal ('off').
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[2.97] Upper Quadrant Semaphore Distant Signal ('off').
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