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London Underground Signalling - 1
A description of signalling on London Underground. This is the first page. Links to other pages are as follows:To London Underground Signalling - 2
To London Underground Signalling - 3
To Victoria Line ATO page
To the Home Page.
Contents
Basics - Automatic Signal Operation - Overlaps - Multi Home Signals - Links to articles on other metro system's signalling
Basics
Railway signalling is based on a simple system 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 accrodingly. The locations of the block boundaries are fixed and so are the signals. The signalling on London Underground is based on this so-called "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.
Fig. 1 London Underground
automatic signal (on the left) with its trainstop in the raised position,
placed at the right hand side of the track. Photo taken at Ruislip Manor
(Metropolitan).
Click on image for full size view.
The photo above shows a typical arrangement of an automatic signal and trainstop. 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.
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".
Fig. 2 Insulated block joints
in 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.
Photo taken at Rayners Lane (Metropolitan).
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".
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.
Fig. 3 Diagram of automatic
signals protecting 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 the 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.
Fig. 4 Diagram showing the
change of signal aspect as a train enters a block. 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.
Fig. 5 Diagram showing the
change of signal aspect of signal A125 as a train enters a second block. The signal
(A123)
protecting the first block is still showing a stop aspect because part of the train is
still in the block.
Overlaps
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.
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 only if the second train stops at the signal. 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 hits 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.
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.
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 would therefore ensure Signal A125 shows a red aspect in the case shown in Fig 9 below.
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 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.
Multi Home Signals
The biggest constraint on throughput for a rapid transit railway like the London Underground is the time spent in stations. If a train waits in a station too long, the following 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.
The normal station signals are laid out as shown in the diagram below.
Fig 10: Diagram of the standard
station signals showing the location of the full speed overlaps for each signal.
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, 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 as the train in front is leaving. One way of doing this is to shorten the overlap of Signal A125.
Fig 11: Diagram of station
signals showing the short overlap for the starting signal A125.
In Fig 11, 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. Even so, 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.
Fig 12: Diagram of multi home
signals showing the additional signals and track circuits required.
When the train in the platform moves 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 13 below). A123c will clear as the train clears the overlap of signal A125. By this means, a following train can approach the platform as the train ahead leaves, as shown below and several valuable seconds are saved.
Fig 13: Diagram of multi home
signals showing how the following train approaches the platform as the preceding train
leaves.
Links to articles on other Metro system's signalling
To London Underground Signalling - 2
To London Underground Signalling - 3
To Victoria Line ATO page
To the Top of this page
To the Home Page.
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Updated 9 March 2002
