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Signalling into Terminal Platforms

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Little attempt was made during the early years to warn drivers of their approach to a buffer stop at the end of a terminal platform line. Indeed, some terminal stations had no signals whatsoever, such as the simple one-platform stations at the end of branch lines worked on the "One Engine in Steam" principle. Drivers would be aware of the presence of the station from their route knowledge and of the need to stop, just as they would at any other station along the line, and speeds on such lines were generally low anyway. Even at the larger terminals with full signalling, the view was held that signals were only intended to warn of temporary obstructions such as another train and not a permanent obstruction like a buffer stop. If the line was clear all the way to the buffer stop, then all the signals could be cleared, including the distant signal, so that trains had an unrestricted aspect sequence running into the platform (figure 1). If, however, the train was routed into a platform line that was significantly shorter than other platforms at the same station, the distant signal remained in the 'on' position. Cleared stop signals exhibited an indication of route (e.g. platform number) where appropriate.

Figure 1
Fig. 1: Unrestricted aspect sequence to buffer stop.

The practice of working terminal platform lines permissively, i.e. admitting a second train into a line that is already partially occupied, originated on the London Brighton & South Coast Railway in 1878. A signal controlling entry into a permissive platform line has to inform the driver whether the line ahead is clear or occupied, which the LB&SCR achieved by mounting a fishtailed distant arm below the stop arm, on the same post. If the platform line was clear, then both arms were cleared (figure 2). If, however, the platform line was partially occupied, then only the stop arm was lowered, with the distant arm remaining 'on' (figure 3). The Caledonian Railway adopted the same practice at important terminal stations, using a ringed distant arm, referred to as a 'precaution signal'.

Figure 2
Fig. 2: Platform line clear.
Figure 3
Fig. 3: Platform line partially occupied.

The drawback with the above arrangement was that trains were being routinely admitted into an occupied line under the authority of a cleared main signal. Later practice was to provide a subsidiary calling-on signal for permissive working. If the platform line is clear, the main semaphore arm is cleared (figure 4), and if the line is occupied, the calling-on arm is cleared once the approaching train has been brought almost to a stand (figure 5). This soon became the standard practice at mechanically signalled stations and remains so today.

Figure 4
Fig. 4: Platform line clear.
Figure 5
Fig. 5: Platform line partially occupied.

The London Victoria terminus of the South Eastern & Chatham Railway had been resignalled with three-position semaphore signals in January 1920. The platform entry signals, which were restricted to operating only in the horizontal and 45° positions, were provided with subsidiary disc signals for permissive working. In September 1924, the Southern Railway altered the signalling, replicating the practice of the former LB&SCR by using a single main signal arm that was cleared to 90° to indicate that the platform line was clear (figure 6), or to 45° if it was partially occupied (figure 7). The subsidiary signals were retained to admit a locomotive into a fully occupied platform.

Figure 6
Fig. 6: Platform line clear.
Figure 7
Fig. 7: Platform line partially occupied.

From March 1926, the colour light signalling schemes of the Southern Railway (and to a lesser extent the LMS) perpetuated this practice with equivalent colour light aspects, the platform entry signals showing a green aspect if the line was clear (figure 8) or a yellow aspect if it was partially occupied (figure 9).

Figure 8
Fig. 8: Platform line clear.
Figure 9
Fig. 9: Platform line partially occupied.

On other railways, a colour light signal reading into a terminal platform line only showed a main proceed aspect if the line was clear to the buffer stop. Usually a green aspect was exhibited, or alternatively a yellow aspect where applicable to a platform line that was significantly shorter than other platforms at the same station. If a platform line was occupied, the signal displayed a red aspect together with a cleared subsidiary signal to admit another train (figure 10).

Figure 10
Fig. 10: Subsidiary signal (e.g. position light type).

Following the accident at Moorgate on 28 February 1975, when a loaded passenger train ran at speed through the buffer stop, the practice of allowing an unrestricted aspect sequence into a terminal platform line became untenable. Semaphore distant signals approaching terminal stations, if still capable of being cleared, were permanently fixed at 'caution' (figure 11). Colour light signals reading into terminal platform lines were altered to show a yellow rather than a green aspect when the line was clear to the buffer stop (figure 12). If necessary to ensure adequate braking distance, the aspect was approach released from red. These altered aspect sequences had the added benefit that the AWS, where fitted, always gave the driver a 'warning' indication on the approach to the station.

Figure 11
Fig. 11: Semaphore distant signal fixed at 'caution'.
Figure 12
Fig. 12: Colour light signals (e.g. four-aspect sequence).

Providing a cautionary aspect sequence into signalled terminal stations created an anomaly in that drivers still had no warning when approaching the buffer stop at the end of a branch line worked by One Train Working or similar. This was remedied during the late 1970s and early 1980s by the provision of fixed distant signals, usually in the form of reflectorised distant boards, on the approaches to those stations, at braking distance to the buffer stops (figure 13).

Figure 13
Fig. 13: Reflectorised Distant Board.

Station Yard Working

The Rule Book included provision, under 'station yard working', for signalling a train into an occupied platform line by clearance of the main signal where no subsidiary signal was provided. The train was first brought to a stand at the signal controlling entry to the occupied platform. After clearing the signal, the signalman exhibited a green handsignal to the driver as a warning that the line ahead may be occupied. If the driver was unable to see a handsignal because the train would not pass the signal box, he had to understand that the line ahead may be occupied if his train was brought to a stand at the signal controlling entry to the platform.

Lime Street Control

Many terminal stations in colour light signalled areas have 'Lime Street Control', originally provided at Liverpool Lime Street station as part of the resignalling in January 1948. Its purpose is to prevent a second train being signalled into an occupied platform line when there is insufficient space to accommodate it. To implement this control, 'measuring' track circuits of specific lengths are provided in the platform lines and on the immediate approach to any main signal reading into them. By comparing the occupancy or otherwise of these track sections, the subsidiary signal controlling entry into the platform line can be prevented from clearing unless it is determined that an approaching train is short enough to fit inside a partially occupied platform line, or if the line is fully occupied that it does not exceed the length of two locomotives.