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Section 2: Main Signals

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There were situations in semaphore signalled areas where due to lack of space it was not possible to accommodate a semaphore arm. In such cases it would be necessary to dispense with the semaphore arm and provide a signal that displayed a red or green light indication by day or night. This creates a problem though, because with colour light practice a green light cannot be followed by a red light (in normal working) but with semaphore signalling it can. To overcome the risk of confusion, the LMS would fit such signals with a white opal marker bearing a black ring [2.111]. The marker was illuminated only when the signal was exhibiting a green aspect, and where practicable only if the signal ahead was at 'danger'. The marker advised the driver that the signal was acting like a semaphore signal and to be prepared to find the next signal at 'danger'.

[2.111] Two-aspect signal at 'clear' with illuminated marker below.

On 28 February 1975 an accident occurred at Moorgate, which at the time was on the London Underground's Northern City Line. A loaded passenger train ran at speed through the buffer stop and into the tunnel end wall. As a result, British Rail decided that in future all colour light signals reading into terminal platform lines would show a yellow (see [2.93]) rather than a green aspect (see [2.94]) when the line was clear to the buffer stop.

During bridge construction work on the Riverside branch (Newcastle) in 1977, the site of the work was protected in each direction by a stop board reading "Stop - Await instructions" (see Section 20). An advance warning board comprising a yellow disc on a white background [2.112] was provided on the Up Branch line to act as a fixed distant signal for the stop board ahead.

[2.112] Advance Warning Board.

The Liverpool Loop line, opened in 1977, was provided with two-aspect signalling similar to that which was already in widespread use on the London Underground. With two-aspect signalling, there are two types of signal, stop signals and repeater signals. A stop signal shows a red light (see [2.92]) for "Danger - Stop" or a green light (see [2.94]) for "Clear - Proceed". Where sighting is good, drivers are expected to be able to stop at a 'Danger' signal with no advance warning. Where sighting of a stop signal is restricted, a repeater signal will be provided in rear. A repeater signal shows a yellow light (see [2.93]) for "Caution - Be prepared to find next signal at Danger" or a green light (see [2.94]) for "Clear - Proceed". A repeater signal for a stop signal in advance may be co-located with the preceding stop signal. Where this occurs, the repeater signal is mounted below the stop signal, so that the "Caution" aspect comprises a green light over a yellow light [2.113] and the "Clear" aspect is green over green [2.114]. When the stop signal is displaying a red aspect, no light is shown in the repeater signal below, in contrast to the combined stop and distant colour light signals that once existed on the GWR (see [2.102]).

[2.113] Green Aspect over Yellow Aspect (Liverpool Loop line).	[2.114] Green Aspect over Green Aspect (Liverpool Loop line).

With the introduction of the Class 253 & 254 'High Speed Trains' (or 'InterCity 125s') to British Rail, it was found necessary to introduce two additional signal aspects. The new flashing aspects were designed to give the driver advance warning that the junction signal ahead (see Section 6) is showing a 'proceed' aspect applicable to the diverging route. Where there is more than one diverging route, the flashing aspects only refer to the highest speed divergence(s). A flashing single yellow aspect [2.115] means "Preliminary Caution - Be prepared to find the next signal displaying one yellow light, with a route indication for the highest speed diverging route". Where provided, the flashing double yellow aspect [2.116] means "Proceed - Next signal displaying flashing single yellow" and is an indication of a diverging route ahead of the next but one signal. The new flashing aspects were introduced from 1978 on all regions except the Southern, initially.

[2.115] Flashing Single Yellow Aspect.	[2.116] Flashing Double Yellow Aspect.

An alternative form of fixed distant signal appeared on the scene at Blaenau Ffestiniog. This was a black board bearing an image of a yellow distant arm [2.117]. Three self-luminous 'Betalights' were fitted to the front of the arm to give an indication during darkness. A problem with the Betalights was that they are radioactive and prone to vandalism.

[2.117] Fixed Distant Board with Betalights.

In the early 1980s, a 'low cost signalling committee' was formed, tasked with finding cheaper means of signalling lightly-used lines. One thing that came out of this was the development of a reflectorised notice board style equivalent of the fixed distant signal. Some experimental boards were manufactured [2.118 - 2.121]. One board [2.118] was similar in appearance to the 'Betalight' distant board (see [2.117]). The style of board that was chosen to become standard [2.121] was first put into use in 1983.

[2.118] Experimental Reflectorised Distant Board.	[2.119] Experimental Reflectorised Distant Board.	[2.120] Experimental Reflectorised Distant Board.	[2.121] Experimental Reflectorised Distant Board (subsequently adopted as standard).

To facilitate special test runs of the new Class 91 electric trains on the East Coast Main Line at 140 m.p.h., it was necessary to introduce a new signal aspect. On part of the route, between Peterborough and Stoke, signals were altered in 1988 to be capable of displaying a flashing green aspect [2.122]. For the test trains, a flashing green aspect gives authority to exceed 125 m.p.h., and a steady green (see [2.94]) means reduce to 125 m.p.h. For all other trains, the flashing green means the same as a steady green.

[2.122] Flashing Green Aspect.

Britain's first high speed line was the Channel Tunnel Rail Link (CTRL), the first section of which opened in 2003. The completed line runs between London and the Channel Tunnel. High speed lines do not have lineside signals, because a driver could not reasonably be expected to read a signal aspect at high speed. Instead, cab signalling is used, with fixed markers placed at the lineside to mark the start of each block section. The CTRL is signalled with a cab signalling system called TVM. This is the same system as used in the Channel Tunnel itself, as well as on the high speed lines in France, where high speed lines had been in operation for many years beforehand. The letters "TVM" stand for 'Transmission Voie-Machine', which means 'Track to Train Transmission'. The British block markers are based on the French 'repères', being a yellow triangle on a blue square background [2.123]. The apex of the triangle points towards the line to which it applies. The marker indicates to the driver the position at which the train must come to a stand when a 'stop' indication is shown on the cab signalling display.

[2.123] TVM Block Marker (e.g. applies to the line on the right).

The European Rail Traffic Management System (ERTMS) is a signalling and management system developed with the backing of the European Union. The objective of ERTMS is to allow seamless operation ("interoperability") across international borders without the problem of having incompatible signalling systems on either side. ERTMS may be overlaid onto ordinary lineside signalling. Alternatively, it may function solely as cab signalling. The standard form of block marker for ERTMS lines with cab signalling is similar to a TVM block marker (see [2.123]), with the triangle replaced by an arrow [2.124]. The arrow points towards the line to which the marker applies. In the unusual situation where a block marker is mounted directly over the line to which it applies, the arrow points down [2.125].

[2.124] ERTMS Block Marker (e.g. applies to the line on the right).	[2.125] ERTMS Block Marker applicable to the line below.

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