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Colour Light Signal Identification

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Signal PU106 at Fort Matilda
Fig. 1: A signal with identification PU106.

Introduction

This page describes the rules that govern the identities of signals in colour light installations, as presented on their identification plates. In general, a signal identity consists of an alphabetical prefix code followed by a number. Additionally, some types of signals may be identified by a suffix. This page does not consider the rules that determine the numerical component or the identification of colour light signals in mechanically signalled areas.

A controlled stop signal is identified by a prefix code followed by a number unique to that prefix, e.g. N24. The prefix code usually identifies the controlling signal box and normally consists of one or two letters or, exceptionally, three. Occasionally, the last character in the prefix code may be a numeral. The prefix is usually derived from the signal box's name, for example:

In modern practice, automatic (and semi-automatic) signals and distant signals are numbered in the same format as controlled stop signals. Any deviations from this format will be noted below.

London and North Eastern Railway

In the case of automatic (and semi-automatic) signals, the prefix letter referred not to the supervising signal box, but to the direction of travel. The number was related to the mileage. In the North Eastern Area, this was the lower of the two mileposts that the signal was situated between, while in the Southern Area it related to the previous milepost in the direction of travel. For example, U2 (in the North Eastern Area) identified an automatic signal located between mileposts 2 and 3 on the Up Main line. If a single mile contained more than one automatic signal applicable to the same line, subsequent signals were suffixed B, C, etc. in the direction of travel. For example, D5B identified the second automatic signal located between mileposts 5 and 6 on the Down Main line.

In the North Eastern Area, a suffix was added to identify automatic signals on lines other than the Main lines. For example, U31S was an automatic signal located between mileposts 31 and 32 on the Up Slow line, and D5BS was the second automatic signal between mileposts 5 and 6 on the Down Slow line. In the Southern Area, an additional prefix letter identified the line. For example, DL19 was an automatic signal located between mileposts 19 and 20 on the Down Local line and UT20B was the second automatic signal between mileposts 20 and 19 on the Up Through line.

Distant signals were identified by an additional prefix letter R before the usual prefix code, e.g. RS112 was the distant signal in rear of stop signal number S112.

London Midland and Scottish Railway

The LMS did not generally plate its signals. An exception was the signalling on the Watford 'New' Line, where the prefix letters of automatic signals were derived from the name of the railway station at which they were located or, if between stations, the name of the next station in advance. 'Repeater' (distant) signals were identified by an additional prefix letter R before the usual prefix code, e.g. RWB5 was the distant signal in rear of stop signal number WB5.

Great Western Railway

The GWR did not generally plate its signals.

Southern Railway

The Southern Railway had a peculiar system to derive the signal box prefix code. To begin with, single letter codes were used, following in alphabetical sequence, e.g. D for Borough Market Jcn. signal box, E for London Bridge signal box and F for Bricklayers' Arms Jcn. signal box. From 1932, new codes included an additional letter C, E or W, which referred to the Central, Eastern or Western Section, respectively. This additional letter could be placed first or last in the prefix to obtain two distinct codes. Some examples, on the Central Section, were CK for Balcombe Tunnel signal box, CL for Three Bridges signal box and CM for Gatwick signal box, also KC for Penge West signal box, LC for Forest Hill signal box and MC for New Cross Gate signal box. Some three-letter examples, on the Western Section, were WBO for Barnes signal box, WBP for White Hart Crossing signal box and WBQ for Mortlake signal box.

Automatic signals were prefixed A, CA and WA on the Eastern, Central and Western Sections, respectively.

Distant signals were identified by the suffix R, e.g. CA54R was the distant signal in rear of stop signal number CA54.

BR Scottish Region

Automatic and semi-automatic signals were numbered in the same manner as controlled stop signals.

From 1976, an additional letter was added to the prefix code of signals in a remote interlocking area; for example, the code PB was for signals in the Bo'ness relay room area, which was remotely controlled from Polmont signal box, whereas P on its own was for signals located in the Polmont relay room area. In S.S.I. schemes, a second letter identified the interlocking area, which could be the same as the first letter, e.g. the code YY identifies signals in the Yoker S.S.I. area, controlled from Yoker Signalling Centre.

Distant signals were identified by the suffix R, e.g. W34R was the distant signal in rear of stop signal number W34. Banner repeater signals were given an additional prefix letter R after the signal box code, e.g. EVR412.

BR North Eastern Region

The North Eastern Region continued to number all signals in accordance with former LNER (North Eastern Area) practice until c.1967.

BR Eastern Region

The Eastern Region continued to number all signals in accordance with former LNER (Southern Area) practice until c.1967.

BR London Midland Region

Automatic and semi-automatic signals were numbered in the same manner as controlled stop signals.

Distant signals and banner repeater signals were not usually plated. Where plated, distant signals had an additional prefix letter R after the signal box code, e.g. GAR101. Inner and outer distant signals were suffixed R1 and R2, respectively. Banner repeater signals, where plated, had an additional prefix letter R either before or after the signal box code, e.g. RWL201 or WNR81.

BR Western Region

Automatic and semi-automatic signals were prefixed with letters referring to the line to which the signal applied. The number referred to the lower of the two mileposts that the signal was situated between; for example, DB105 was an automatic signal located between mileposts 105 and 106 on the Down Badminton line. If a single mile contained more than one automatic signal applicable to the same line, each was suffixed A, B, etc. in the direction of travel. For example, UFM1B identified the second automatic signal located between mileposts 1 and 2 on the Up Filton Main line.

Distant signals and banner repeater signals were identified by the suffix R, e.g. DM69R was the distant signal in rear of stop signal number DM69.

On the Newport resignalling scheme, main controlled signals on the Main or Relief lines were prefixed N in the usual manner. Shunting signals and all other main controlled signals were prefixed according to their former control area, however; e.g. the prefix code B referred to the Ebbw Jcn. area.

On the Reading resignalling scheme, main signals that were controlled from a level crossing ground frame had a two-letter prefix. The first letter R indicated Reading signal box and the second letter denoted the crossing frame. For example, RK103 was a signal controlled from Kintbury Crossing ground frame.

In 1981, bi-directional signalling was provided between Hullavington and Wootton Bassett, in the Swindon signal box area. Wrong-direction automatic signals were given an additional prefix letter after the signal box code, denoting the direction of travel. For example, SND4 is a Down direction automatic signal on the Up line and SNU3 is an Up direction automatic signal on the Down line.

In 1989, simplified bi-directional signalling (SIMBIDS) was introduced between Thingley Junction and North Somerset Junction, in the Bristol signal box area. Wrong-direction automatic signals were prefixed MD on the Up Main line (Down direction) and MU on the Down Main line (Up direction), therefore corresponding with the prefix codes of the normal-direction automatic signals on the adjacent line but with the letters transposed.

BR Southern Region

The Southern Region continued to number its automatic signals in accordance with former Southern Railway practice until August 1974, after which they were numbered in the same manner as controlled stop signals.

On the South Eastern Division, distant signals were identified by the suffix R.

On the Central and South Western Divisions, distant signals were identified by an additional prefix letter R or D before the usual prefix code. For example, DWFQ7 was the distant signal in rear of stop signal number WFQ7 and RWA403 was the distant signal in rear of automatic signal number WA403. Inner and outer distants were prefixed D1 and D2, respectively.

Banner repeater signals were identified by the suffix R.

In the Dartford resignalling scheme of 1970/1971, semi-automatic signals controlled from a ground frame had an additional prefix letter. The usual prefix code indicated the signal box from which the ground frame was controlled and the additional letter, the ground frame from which the signal was controlled. For example, DJ539 was a signal in the Dartford signal box area controlled from ground frame "J" and CXA575 was a signal in the Cuxton signal box area controlled from ground frame "A".

Victoria Signalling Centre had two control panels, each controlling a large area. The second letter of the prefix code identified the panel concerned, i.e. VC for the Central panel, and VS for the South Eastern panel.

On certain schemes (e.g. Three Bridges, 1983), the suffix X denoted a wrong-direction signal on a bi-directional line, e.g. T367X. The X preceded any other suffix letters denoting distant signals, e.g. AD168XR and T423XRR.

Signal YS154 at Airdrie
Fig. 2: Distant signal with 'delta' plate.

British Rail Standard Practice

The British Railways Board laid down some rules that were meant to standardise signal numbering across all regions.

Automatic and semi-automatic signals were to be numbered in the same manner as controlled stop signals.

Distant signals (most restrictive aspect yellow) were identified by the number of the stop signal to which they referred, followed by the suffix letter R. Outer distant signals (most restrictive aspect double yellow) carried the suffix RR. There was no requirement for banner repeater signals to carry identification plates.

From 1992, banner repeater signals would take the number of the signal to which they referred, with the addition of the suffix BR, while co-acting signals would take the number of the primary signal but with the suffix CA.

Railtrack

In 1996, the 'delta' plate, bearing a triangular symbol, was introduced as a means of identifying colour light distant signals without reference to the signal's number. In most new schemes since, distant signals have been uniquely numbered, without a suffix. On some schemes, however, distant signals continued to be numbered with the R or RR suffix in addition to the delta plate, to maintain consistency with adjacent areas.

Network Rail

From 2004, the Midlands numbering strategy introduced a system whereby the prefix letters relate to the portion of route and not to the controlling signal box. Some example prefix codes are:

The same method of allocating signal prefix codes has been adopted on some other routes but it is not being applied throughout the entire network.

Channel Tunnel Rail Link

On the CTRL, controlled signals, absolute stop markers and shunt markers are prefixed with the letters AF, for Ashford Centre. Leading zeros are added to the numbers as necessary so that they always comprise three digits, e.g. AF012.

Automatic signals and stop & proceed markers are prefixed according to the line to which they apply and the number refers to the lower of the two kilometre points that they are situated between, leading zeros being added as necessary so that there are three digits. For example, U087 is located between kilometre points 87 and 88 on the Up CTRL line. If a single kilometre contains more than one signal/marker applicable to the same line, each is suffixed A, B, etc. in the direction of travel. For example, D048A identifies the first signal/marker located between kilometre points 48 and 49 on the Down CTRL line.

A second prefix letter is added to identify automatic signals or stop & proceed markers on lines other than the main CTRL lines, e.g. DW203 on the Down Waterloo Connection.

A wrong-direction stop & proceed marker is suffixed with the letter X, e.g. U106X applies in the Down direction on the Up CTRL line.