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Section 2: Main Signals

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The provision of yellow arms and lights on semaphore distant signals became a requirement of the Ministry of Transport in 1925. This rendered superfluous the Coligny-Welch lamps used on some railways (see [2.64 - 2.67]). There was some objection to their removal, however, and in some cases they were retained for a period after yellow arms had been fitted [2.98 & 2.99]. There was even a suggestion that the colour of the chevron light should be altered from white to yellow when the new arms were fitted, but this was not done. The Southern Railway finally abolished its Coligny-Welch lamps in 1927.

[2.98] Semaphore Distant Signal with Yellow Arm and Coligny-Welch Lamp ('on').	[2.99] Semaphore Distant Signal with Yellow Arm and Coligny-Welch Lamp ('off').

The adoption of the upper quadrant semaphore signal (see [2.96 & 2.97]) was agreed to at a meeting of the Railway Clearing House on 20 July 1926. The 1928 issue of the MoT 'Requirements' would no longer dictate that semaphore arms must operate in the lower quadrant, as the previous issue had done in 1925. The Great Western Railway (and subsequently the Western Region of B.R.) resisted any move towards adopting the upper quadrant signal and continued to install lower quadrant signals, almost exclusively.

The Southern Railway perpetuated their special use of main signals for entering the platform roads at terminus stations, which it inherited from the LB&SCR. Where a colour light signal was used in this situation, a yellow aspect (see [2.93]) meant that the platform road was already partially occupied, while a green aspect (see [2.94]) meant that the platform road was unoccupied (i.e. clear to the buffer stop).

In 1929, the arms of banner distant signals on the Southern Railway were altered from red to yellow and the background colour was changed to black [2.100 & 2.101].

[2.100] Banner Distant Signal with Yellow Arm ('on') (SR).	[2.101] Banner Distant Signal with Yellow Arm ('off') (SR).

The Great Western Railway chose not to install colour light signalling of the form that was becoming standard on the SR, LMS and LNER. Instead, colour light signals installed on the GWR, from 1931, displayed aspects that were the same as shown by semaphore signals at night. Thus a colour light stop signal could display either red for 'danger' (see [2.92]) or green for 'clear' (see [2.94]). A colour light distant signal showed either yellow for 'caution' (see [2.93]) or green for 'clear' (see [2.94]). The principles of semaphore signalling were perpetuated to the extent that a stop and distant signal could be mounted together on one post, with the stop signal head positioned above the distant. A 'danger' aspect was red over yellow [2.102], a 'caution' aspect was green over yellow [2.103], and a 'clear' aspect was green over green [2.104].

[2.102] Red Aspect over Yellow Aspect (GWR).	[2.103] Green Aspect over Yellow Aspect (GWR).	[2.104] Green Aspect over Green Aspect (GWR).

A.F. Bound, in his new position as Signal Engineer of the LMS, devised a system of colour light signalling that was installed from June 1932 between Camden and Watford Junction ('New' Line) and between Bromley and Upminster. The main aspects used were the red aspect (see [2.92]), yellow aspect (see [2.93]), green aspect (see [2.94]) and a small number of double yellow aspects (see [2.95]). A novel feature, copied from North American practice, was the provision of marker lights, the principal purpose of which was to identify the signals as being within 'multiple-aspect colour light' territory. Any signal that read towards a semaphore signal or a London Transport signal was not provided with a marker light. The marker light comprised a red light mounted lower down the post, below the main aspect. It was only illuminated while the main signal was displaying a 'danger' aspect [2.105], except that it was extinguished when a subsidiary aspect was displayed (see Section 4). Where the marker light was offset to the left [2.106], the driver was permitted to proceed cautiously past the signal at 'danger', after waiting for one minute.

[2.105] Stop Signal with Marker Light (LMS).	[2.106] Repeater Signal with Marker Light (LMS).

In July 1932, Bound's experimental colour light signalling scheme was commissioned in the Mirfield area on the LMS. There were five main aspects. In addition to the red aspect (see [2.92]), yellow aspect (see [2.93]), double yellow aspect (see [2.95]) and green aspect (see [2.94]), the fifth aspect was yellow over green [2.107]. Inserted into the normal aspect sequence between the green aspect and the double yellow aspect, the yellow over green aspect meant "attention - pass second signal at restricted speed". Having five aspects allowed the signals to be spaced closer together.

[2.107] Yellow over Green Aspect (LMS).

Red marker lights were also used in the Mirfield scheme. The marker light was positioned on the post below the main aspect [2.105 & 2.108 - 2.110], except that signals which read towards a semaphore signal did not have marker lights. The marker light remained illuminated except when the main signal was displaying a 'clear' aspect (see [2.94]), or when a shunting or subsidiary aspect was displayed.

[2.108] Single Yellow Aspect with Marker Light (LMS).	[2.109] Double Yellow Aspect with Marker Light (LMS).	[2.110] Yellow over Green Aspect with Marker Light (LMS).

In 1933, discussions commenced on the standardisation of colour light signals, at the instigation of the Ministry of Transport.

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