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Approach Light Signals (Semaphore to Colour Light Transition)

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There have been a few alternative methods of transitioning from semaphore signal territory into colour light signal territory. No special arrangements are needed where the transition from one form of signal to the other is separated by around a mile or more, providing a clear gap between the two different types of signals. If, however, the colour light territory immediately follows the semaphore territory, then the question arises as to what form the last signal at the exit from the semaphore territory should take, considering the need to display appropriate 'caution' aspects on the approach to the first colour light signals in the sequence.

One option is to install a motor-operated semaphore distant arm below the last semaphore stop signal. Another option is to replace the last semaphore stop signal with a colour light stop signal equipped to display the desired 'caution' aspects relating to the colour light signals ahead. One of the original methods commonly employed entailed the provision of a hybrid form of signal comprising a colour light distant signal mounted underneath a semaphore stop arm. The Southern Railway referred to these colour light distant signals marking the approaches to colour light territory as 'approach light' signals (not to be confused with 'approach lighting' or 'approach lit' signals). Whilst the use of these signals was not solely restricted to that company's lines, information on this page principally relates to Southern Railway practice. The semaphore signals concerned may be of the lower or upper quadrant variety.

Figure 1
Fig. 1: Approach Light Signals.

Where provided, an approach light signal was mounted underneath the last semaphore signal at the exit from the semaphore signal territory and it gave drivers an indication of the aspect displayed by the first colour light signal in the sequence ahead. The earliest approach light signals could display only single yellow ('caution') or green ('clear') aspects.

Figure 2
Fig. 2: Aspect Sequence.

Where necessary to provide sufficient braking distance approaching the first colour light stop signal, a second approach light signal was mounted below the semaphore stop signal preceding the last semaphore stop signal. This arrangement included what were referred to as inner and outer approach light signals. Originally, both were arranged to show the same aspect at the same time (single yellow or green).

Figure 3
Fig. 3: Aspect Sequence - Inner and Outer Approach Light Signals.

The first approach light signals were brought into use in the south of London on the Southern Railway in March 1926. It was quickly realised that the approach lights overpowered the weaker lights associated with the semaphore arms above them, introducing the risk that they could be inadvertently passed at 'danger'. After just a few days in service, the approach lights were altered to display no aspect while the semaphore stop arm above was in the 'danger' position.

Figure 4
Fig. 4: Approach Light Signal unlit while semaphore stop arm at 'danger'.

From 1929, the practice was adopted of maintaining an outer approach light unlit while the semaphore stop arm above the corresponding inner approach light was at 'danger', even though the semaphore arm above the outer approach light was in the 'off' position. The approach lights were therefore only illuminated when the line was clear at least as far as the first colour light signal.

Figure 5
Fig. 5: Aspect Sequence - Inner and Outer Approach Light Signals.

Approach light signals capable of showing a double yellow aspect first appeared in 1929, enabling the display of a normal colour light aspect sequence leading into four-aspect signalling territory. During 1929, several existing two-lens approach lights were replaced with three-lens heads equipped with two yellow lights.

Figure 6
Fig. 6: Approach Light Signal displaying Double Yellow Aspect.

As a consequence of the introduction of approach lights capable of displaying a double yellow aspect, it ceased being necessary for inner and outer approach lights to both display the same aspect at the same time.

Figure 7
Fig. 7: Aspect Sequence - Inner and Outer Approach Light Signals.

From 1939, it became the practice to exhibit no light alongside a semaphore stop arm in the 'clear' position, if an approach light signal was provided below. This was achieved by replacing the green spectacle in the semaphore arm with a blinder, leaving just the red spectacle in place.

Figure 8
Fig. 8: No Light in Semaphore Signal when 'off'.

In the absence of a green light alongside a semaphore stop arm in the 'clear' position, the approach light became the only means of displaying a 'proceed' aspect to drivers during darkness. It was therefore essential for the approach light to always show an aspect when the semaphore stop arm above was 'off'. As a result, the former practice of extinguishing an outer approach light when the stop signal ahead was at 'danger' had to be discontinued.

Figure 9
Fig. 9: Aspect Sequence - Inner and Outer Approach Light Signals.

The provision of approach light signals remained as a permitted option within the signalling standards until 1995. The last remaining example was at Bognor Regis.