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Speed Control Signals

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Speed Control Signalling

Some locations on London Underground are provided with speed control signalling.  Trains are forced to reduce speed by red signals which do not clear until the train speed has been reduced and confirmed.  This page describes the systems formerly and presently used on London Underground.

Contents

Speed Control Signalling - Driver's View of Speed Control Signalling - Permissive Signals


Speed Control Signalling

Many ideas for improving the movement of trains through stations were originally imported from the USA and were adopted by London Underground from the 1920s.  One such idea was Multi Home signals, which were introduced as early as 1908 and, in the late 1930s, speed control signals were introduced at some locations.  During the 1940s and 50s they were installed on many lines.  Most have been removed but there are still examples of them still around today. 

The principle of operation is that a signal with speed control normally displays a red aspect until a train approaches it at the reduced speed required.  The speed is checked by a short timing-section track circuit.  The time taken by the train passing through this section must be great enough to indicate that the reduced speed is being achieved or the signal will not clear.   If the speed is too high, the signal will remain at red until the train has stopped at the signal, then it will clear automatically.  An example of this is the approach to Archway (SB Northern Line) where there is a 30 mi/h limit controlled by speed control signals.  The approach to Highgate was also provided with a speed control signal in the early 1960s.

Speed control at stations was introduced at a number of locations in the late 1940s.  This was really a sophisticated system of multi home signals, as shown in the next diagram.

sig block sc1.gif (10757 bytes)

Fig 1:  Diagram of speed control at stations introduced at a number of locations in the late 1940s and withdrawn in the 1970s.  

When there is no train in the platform, all signals will show green and the speed limit signs will be switched off.  If there is a train in the platform, all signals are red and the two speed limit signs are lit.   The full speed overlap for the platform berth starts at signal A123a.  To allow the train to draw closer to the platform, the speed must be reduced to 25 mi/h and signal A123a will clear.  If the train speed is then reduced to 20 mi/h, signal A123b will clear.  By this time, the train in the platform should have begun to leave, allowing signals A123c, d, e and f to clear in sequence, allowing the following train to run into the platform at about 20 mi/h.

This system was installed on the Piccadilly Line between Green Park and Kings Cross in 1948 and then on the District between Sloane Square and Mansion House.  The speed detectors on the Piccadilly were inductors set into dummy sections of current rail, while those on the District used track circuits.    Speed control of home signals was installed at a number of other individual locations like Liverpool Street (Central) and Oxford Circus (Bakerloo), where platform times were long.

The system had to be accurately designed and skilfully used by drivers to work properly.  Combined with firm control of passenger loading at the stations involved, it worked very well.  It was withdrawn when the lines were resignalled at a time when traffic levels were falling and train services were being reduced.  There may be a good case for re-introducing it at some locations.

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The Driver's View of Speed Controlled Signalling

Some locations on LU are still provided with a simple version of speed controlled signalling.

Acton Home Sigs1.jpg (46507 bytes)In the photos (link on the left) kindly supplied by "Tube Troll", the sequence of operation of the four home signals approaching Acton Town (Piccadilly Line eastbound platform) from Ealing Common is shown.  

Click on image for full size view and description.

The first of the four signals is speed controlled, the remaining three clear to green as the preceding train leaves the platform.  In this example, there is no illuminated speed limit sign.  Drivers learn how to approach the signal as part of the "road training".

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Permissive Signals

Another form of speed control signal is the permissive signal, or draw up signal.  These are used at locations were there is a very short overlap reading over a junction, at places like Earls Court and Baker Street.  It is designed to repeat the aspect of the starting signal and to force the train to reduce speed if the signal is at red.  It is unique on LU in being a three-aspect signal.  It shows green if the starter is green, red if the starter is red and yellow when the speed of the train is confirmed as reduced to the required level.

 

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