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Automatic Signalling on London Underground

This is the first page of a series on LU signalling.  It is a description of the automatic signalling used on London Underground. Links to other the other signalling pages are listed in the column on the left.


Basics - Automatic Signal Operation - How Automatic Signals Work - Overlaps - Multi Home Signals


Railway signalling is based on a simple principle for keeping trains a safe distance apart.  The principle is that a line is divided into sections called blocks and only one train is allowed into one block at one time.  Each block is protected by a signal at its entrance.  The train driver approaching the signal responds to its stop or go indication and either stops the train at the signal or proceeds into the block accordingly.  The locations of the block boundaries are fixed ("fixed block" signalling) and so are the signals.  The signalling on London Underground is based on this fixed block system.  This is used on all lines including the Victoria and Central Lines, which use also ATO (Automatic Train Operation).  More on the Victoria Line ATO system here.

Each track is divided into sections ranging from a few metres to a few hundred metres long.  The average length is about 300 metres.  The entrance to each block is protected by a colour light signal (we are referring to non-ATO lines here).  To prevent a train proceeding at normal speed into an occupied section, each signal is provided with a mechanical "trainstop" adjacent to the track.   By means of a trip arm on the train, the trainstop will apply the brakes of any train which attempts to run past a stop signal.

Ruislip_Manor_st_small.JPG (2070 bytes)The photo on the left (Fig. 1) shows a typical arrangement of an automatic signal and trainstop. 

Click on image for full size view

The trainstop is lowered when the signal shows a clear (green) aspect.  Compressed air is used to lower the trainstop against the spring pressure used to raise it.  This provides an element of fail safe, should the air supply be lost.  The air is compressed in the traction sub-stations and is distributed alongside the track in an air main.  This can be seen as the silver pipe in the photo above supported on posts at the track side, together with the signalling and power cables.

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Automatic Signal Operation

Each block is equipped with a "track circuit",  a low current circuit passing through the running rails.  This is used to detect the presence of the train.  The circuit is arranged so that if it fails, the signal will show a red aspect.  Track circuits failures and other spurious red indications are the principal cause of "signal failure" announcements so common these days.

Track circuits were first introduced to the UK in 1903 on the Ealing and South Harrow Railway, now part of the Piccadilly Line branch to Rayners Lane.  The system was imported from the USA, where it had been tried on a number of densely used routes.  The track circuits in adjacent block sections were isolated from each other by cutting the rails at the ends of the block and inserting insulated joints.  These became known as "insulated block joints". 

Block joints.JPG (39231 bytes)Fig. 2 on the left shows the insulated block joints in the running rails at the end of a block section.  Note the raised trainstop on the right hand side of the track acting with a red signal (not seen here) to protect the entrance to the next block.

Click on image for full size view

Insulated block joints are a constant source of weakness in that they are prone to failure under intensive use.   In the last ten years, London Underground has been replacing the old track circuits with new ones which do not require block joints to separate them.  These are known as "jointless track circuits".

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How Automatic Signals Work

The following diagrams show the basic operation of automatic signals on London Underground.  Automatic signals are identified by a number preceded by the letter A, e.g. A123.  As long as the block it protects is clear of trains (and everything is working properly) the automatic signal will remain green (Fig 3 below).

sig block1.gif (3823 bytes)

Fig. 3:  Diagram of automatic signals positioned to protect the entrance to block sections.  Note the signals are identified by the A prefix and are numbered like houses, odd numbers on one side and even numbers on the other side.

When a train enters the block, the presence of the train is detected by the track circuit and the signal automatically displays a red aspect as shown below. 

sig block2.gif (3823 bytes)

Fig. 4:  Diagram showing how the signal aspect changes as a train enters a block (Block 2).  Remember that the signal has a trainstop which rises when the signal changes to red.

The signal will remain red all the time the train is in the section.  When the train enters the next section (shown below Fig 5) the signal protecting that section will also change to red.

sig block3.gif (3779 bytes)

Fig. 5:  Diagram showing the aspect change of signal A125 as the train enters the second block.  The signal (A123) protecting the first block is still showing a stop aspect because part of the train is still in Block 2.

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The system so far described shows a simple form of train protection but, in reality, it is more complex.  See what happens if a train stops just past the entrance to the block, as shown in the next diagram.

sig block4.gif (4085 bytes)

Fig. 6:  Diagram showing a second train approaching signal A125.  If Train 2 fails to stop at A125, there is not enough room beyond A125 for the train to stop before it hits the first train standing just beyond the entrance to the block.

The train stopped just inside Block 3 is protected by Signal A125, but only if the second train stops at the signal correctly.  Should it run past the signal, it will get "tripped" by the trainstop of A125.  However, there is insufficient room for the train to stop before it collides with the first train.  The safe braking distance is too short.  To overcome this situation, each signal is moved back a safe braking distance from the entrance to the block.  This distance is called the overlap as shown in Fig 7 below.

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Fig. 7:  Diagram showing how overlaps are provided for signals by placing them a safe braking distance from the entrances to blocks.

The overlap now provides a safe braking distance for any train which overruns a signal so that there is no risk of it colliding with a stopped train in front. Very clever but this arrangement creates a difficult new situation, as shown in the next diagram below.

sig block6.gif (4770 bytes)

Fig. 8:  Diagram showing how a train standing in the overlap of a signal can be in a position with a green signal showing behind it.

Because the overlap has to be of sufficient length to allow a train travelling at normal speed to stop, it can be longer than a train's length.  This could result in a train standing ahead of a signal but that signal would be showing a clear aspect.  Although, in our diagram, the train is still protected by signal A123, it is not considered safe on London Underground to allow a green signal to be displayed behind a train.  Overlaps are therefore track circuited separately and coupled to the block ahead (in this case, Block s) and would therefore ensure Signal A125 shows a red aspect as shown in Fig. 9 below.

sig block7.gif (4719 bytes)

Fig. 9:  Diagram showing how the inclusion of overlap track circuits provides protection for a train standing in the overlap of a signal.

The effect of providing overlap track circuits  and coupling them to the block ahead is that the train has two signals protecting it.  There is always a safe braking distance beyond a red signal for a train standing in any location.

Stamford_Brook_DR_WB_Starter_GG_small.JPG (1744 bytes)A set of photos (see icon on the left) shows how the sequence of automatic signals works as a train passes from one station to another.

Click on image for full size view


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Multi Home Signals

The biggest constraint on throughput for a rapid transit railway like the London Underground is the time spent in stations.  Trains run at frequent intervals (or at least they should) so they are close together.  It follows then, that if a train waits in a station too long, the next train will be delayed and no amount of fancy signalling will help matters.  However, there are some things which can be done to reduce the delay to a following train if the dwell time in the platform is likely to be longer than normal.  One of these is to install multi home signalling.

Traditional station signals are laid out as shown in the diagram below.

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Fig. 11:  Diagram of the standard station signals showing the location of the full speed overlaps for each signal.

In this arrangement, if a train approaches the station home signal, A123, at normal speed and fails to stop, the overlap is of sufficient length to allow it to stop before reaching the train in the platform.  If it does stop normally at the signal, it will have to wait for the train in the platform to leave and clear the overlap of signal A125.  However, some time could be saved if the second train could be allowed to approach the platform once the train in front has started to leave.  One way of doing this is to shorten the overlap of Signal A125.

sig block mh2.gif (5772 bytes)

Fig. 12:  Diagram of station signals showing the short overlap for the starting signal A125.

In Fig. 12, the overlap of A125 has been reduced to a few metres.  Reducing the length of the starting signal overlap allows the signal in rear (A123) to clear sooner so that the next train can run into the platform a few valuable seconds earlier than it could if a full speed overlap had been provided.  

However, the shorter overlap on the starter signal represents a reduction in safety because there is insufficient braking distance should a train run past the signal at full speed.  However, this is unlikely, since most trains will be slowing for a stop at the station anyway.  In case a train has to run through without stopping, a rule (nowadays enforced by a time delay system on the starter at many places) restricts all trains to 5 mi/h when running non stop through stations where they are normally booked to stop.

A further reduction of the time spent waiting at a home signal for a train to clear the platform can be achieved by the use of multi home signals. This is done by installing additional signals and additional track circuits as seen in the next diagram.

sig block mh3.gif (7810 bytes)

Fig. 13:  Diagram of multi home signals showing the additional signals and track circuits required.

When the train in the platform starts to leave and moves forward to clear the first track circuit at about one-third of the platform length, Signal A123a will clear.  At the second track circuit clearance point, at about two-thirds of the platform length, A123b will clear (Fig. 14 below).  A123c will clear as the train clears the overlap of signal A125. 

sig block mh4.gif (7967 bytes)

Fig 14:  Diagram of multi home signals showing how a second train approaches the platform as the preceding train leaves.

By the use of multi home signals, a following train can approach an occupied platform as the train ahead leaves, as shown below and several valuable seconds are saved.  Multi home signals are in use in many places all over London Underground, especially in the central areas where train frequencies are highest.


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