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| Fig. 1: A signal with identification PU106. |
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This page describes the principles that determine 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 signifies 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 identified in the same format as controlled stop signals. Any deviations from this format will be noted below.
The prefix letters of automatic (and semi-automatic) signals' identities referred not to the supervising signal box, but to the direction of travel. The number was derived from 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 signified the applicable 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. For example, RS112 was the distant signal on approach to stop signal number S112.
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 beyond. 'Repeater' (distant) signals were identified by an additional prefix letter R before the usual prefix code. For example, RWB5 was the distant signal on approach to stop signal number WB5.
The GWR did not generally plate its signals.
The Southern Railway implemented a peculiar system to derive the signal box prefix code. To begin with, single letter codes were used, following in alphabetical sequence, for example:
From 1932, new signal box prefix codes on the Central and Western Sections included an additional letter C or W, 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:
A third letter could be inserted in the middle to create a further sequence of codes. Some examples on the Western Section were:
Automatic signals were prefixed A, CA and WA on the Eastern, Central and Western Sections, respectively.
Distant signals were identified by the suffix R. For example, CA54R was the distant signal on approach to stop signal number CA54.
Automatic and semi-automatic signals were identified in the same format 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 identified signals in the Bo'ness relay room area, which was remotely controlled from Polmont signal box, whereas P on its own identified signals located in the Polmont relay room area. In Solid State Interlocking (SSI) schemes, a second letter signified the interlocking area, which could be the same as the first letter. For example, the code YY identifies signals in the Yoker SSI area, controlled from Yoker Signalling Centre.
Distant signals were identified by the suffix R. For example, W34R was the distant signal on approach to stop signal number W34. Banner repeater signals were given an additional prefix letter R after the signal box code, e.g. EVR412.
Signal identification on the North Eastern Region followed former LNER (North Eastern Area) practice until c.1967.
Signal identification on the Eastern Region followed former LNER (Southern Area) practice until c.1967.
Automatic and semi-automatic signals were identified in the same format 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.
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.
Signals that were controlled from a level crossing ground frame but supervised from a remote signal box had an additional prefix letter. The usual prefix code for the supervising signal box was followed by an additional letter signifying the crossing frame. For example, RK103 was a signal controlled from Kintbury Crossing ground frame and supervised from Reading signal box.
Distant signals and banner repeater signals were usually identified by the suffix R. For example, DM69R was the distant signal on approach to stop signal number DM69. Very occasionally, the alternative suffix letter P was used (the letter "P" in electrical signalling nomenclature stands for "repeating").
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. For example, the prefix code B referred to the Ebbw Junction area.
In the early 1980s, bi-directional signalling was provided in the Swindon signal box area on the routes west of Swindon, towards Bristol. Wrong-direction automatic signals were given an additional prefix letter after the signal box code, denoting the direction of travel. For example, SND4 was a Down direction automatic signal on the Up line, and SNU3 was an Up direction automatic signal on the Down line.
In May 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.
Signal identification on the Southern Region generally followed former Southern Railway practice until August 1974.
From 1957, new signal box prefix codes on the South Eastern Division (successor to the Southern Railway's Eastern Section) included an additional letter E. This additional letter could be placed first or last in the prefix to obtain two distinct codes. Some examples were:
A third letter could be inserted in the middle to create a further sequence of codes. Some examples were:
Automatic signals on the South Eastern Division continued to be prefixed with the single letter A.
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 on approach to stop signal number WFQ7, and RWA403 was the distant signal on approach to 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 early 1970s, some semi-automatic signals controlled from a ground frame were given an additional prefix letter. The usual prefix code signified the signal box from which the ground frame was controlled, and the additional letter signified the ground frame that controlled the signal. For example, DJ539 was a signal in the Dartford signal box area controlled from ground frame "J", and YWF299 was a signal in the Basingstoke signal box area controlled from ground frame "F".
Victoria Signalling Centre had two control panels, each controlling a large area. The second letter of the prefix code signified 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.
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| Fig. 2: Distant signal with 'delta' plate. |
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In the 1970s, the British Railways Board specified a common set of principles with the objective of standardising signal numbering across all the regions.
Automatic and semi-automatic signals were to be identified in the same format 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) had the suffix RR. There was no requirement for banner repeater signals to carry identification plates.
From 1992, banner repeater signals were to take the number of the signal to which they referred, with the addition of the suffix BR, while co-acting signals took the number of the primary signal but with the suffix CA added.
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 identity. In most new schemes since, distant signals have been uniquely numbered, without a suffix. On some schemes, however, distant signals continued to be identified with the R or RR suffix in addition to the delta plate, to maintain consistency with adjacent areas.
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.
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. For example, U106X applies in the Down direction on the Up CTRL line.
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