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London Underground Signaling

I am grateful to my good friend Roger Viggers for allowing me to use this piece. The illustrations and captions were added by me and these notes appear in blue.




Visitors to London acquainted mainline signaling practice doubtless find certain aspects of LU's signaling a touch confusing. Signals momentarily showing both red and green aspects or what appears to be a four aspect signal showing green and yellow are not what people brought up on the mainline are used to.

The dual red/green aspect is caused by the associated trainstop arm being slow to drop when the signal goes to green. The section ahead being clear allows the proceed aspect to illuminate but the red stays lit until the trainstop is proved to be fully down. The trainstop arm is held up by springs and forced down by air pressure; some are a bit stiff and slow to move. A cause of trainstops failing to go down fully is rubbish, such as drink cans, becoming trapped underneath.

Trainstop with head raised - the associated signal will be at Danger. You may notice the marks on the white paint which the head is painted with. This does not necessarily indicate that the signal has been overrun - it may be that the signal has failed at some point and it's been necessary for drivers to 'apply the appropriate procedure'

The same trainstop now lowered indicating that the associated signal is showing a Clear (Green) aspect

The apparent green and yellow is in reality two separate signals. The green is the proceed aspect of a stop signal while the yellow is the caution aspect of a repeater (not a distant). When the stop signal shows danger the repeater is completely extinguished. Repeaters are used where sighting distances are restricted and each one is for a designated signal. Sometimes a repeater is for more than one stop signal, but not always the next. To improve sighting in bad visibility additional repeaters are used. These are a miniature yellow/green signal with a white surround marked "Fog Repeater". In multiple aspect area the yellow is replaced by a lunar white lamp with an "F" on the lens.

In the diagrams added below the position of the signal plates is to highlight which relates to the signal and which to the repeater.  In operation the plates are often mounted in the way I have illustrated for the four aspect version.  In practice plates are not necessarily physically attached to the signals - there are many times where the plate is screwed to an adjacent wall or other piece of infrastructure - they are even found 'on the ground' beneath a signal - particularly in covered areas such as tunnels where the signal head is of the tunnel type.

In this schematic the signal is A125 and the repeater beneath it is for signal A127 - hence the signal plate shows R127. The signal A125 is at Danger - therefore the aspect on the repeater is suppressed. The idea is that if a driver is stopped at A125, he does not need to know what the next signal is showing - that only becomes relevant when the signal A125 clears.

Next in the sequence one would normally expect to see is A125 clearing and the repeater showing that A127 is at Danger. This is not necessarily the case of course - there are a number of reasons why this may differ!

On occasions when a train is approaching our signal A125 and it and the following signal A127 are both clear or signals would appear as in this diagram.                                                            

London Underground does also use what appear to be 4 Aspect signals, but these are in fact two, two aspect signals but built onto the same mounting plate.  They are identical in meaning to the examples above. Schematically they would look as follows:


Fog Repeaters appear on many parts of London Underground.  In fact on the Sub Surface Lines (that is the District and Metropolitan) they are now left on at all times as part of London Undergrounds continued programme to drive down the number of SPADs that occur - these are often due to poor siting of signals and the signals in question of have no 'permanent' repeater as described above.


As on the mainline Automatic Signals are controlled by the passage of trains through track circuits. They normally show a green aspect until a train passes or a fault occurs. What on the mainline is called a "controlled signal" is on LU a Semi-Automatic Signal. The normal aspect is danger and it only clears when the signalman, or more likely a programme machine or computer, operates the control. As the train enters the section it automatically returns to red. "Autos" are used on plain track and "Semis" at junctions.

Two types of signal number plates can be seen, black on white for stop signals and black on yellow for repeaters. Semi-automatic signals are numbers with a corresponding lever in the signal cabin or interlocking machine room (relay room) and are prefixed by the cabin code. Automatic signals are sequentially numbered and prefixed by the letter "A". On part of the former City & South London Railway (Northern Line City Branch) the "A" is replaced by an "S".

Signal Plate as attached to an Automatic Signal

                                Signal Plate as attached to the Repeater Signal for A150

          Signal Plate as attached to a Semi-Automatic Signal. 'EC' is the Cabin Code and '3' indicates the lever number.

Signal Plate as attached to the Repeater Signal for EC3

Different sizes of signal are used in tunnels and on open sections. In tunnels the lens is 4 inches diameter with a lens designed to spread the light. 8-inch lenses are used on the surface with a narrow beam for long distance sighting. As an aid at very close range a supplementary lens, (known as a "pig's ear") is fitted at the side of the main lens.

                                                      Tunnel Section Signal Head

    Open Section Signal Head. The 'Pigs Ear' is just visible as the small red light just to the left of the main signal aspect.

Coincidentally both these signals have Rail Gap indicators attached to them. See below for an explanation of their purpose.

The layout of signals is similar to the absolute block system. The station, or junction, is the block post. At the front end of the platform is the Starter. If there is a junction just beyond the platform there may be Advance Starters on each branch. The signals approaching the station are the Home signals, of which there may be a number. At King's Cross on the eastbound Piccadilly Line there used to be five, the first two were speed controlled. Under speed controlled signaling if the driver had not reduced speed to a predetermined level the signal will remain at danger and if passed the train will be tripped resulting in an emergency brake application. As the train in the platform departs the home signals will clear in succession and the following train can enter. The whole system is designed to reduce headways and keep trains moving.

Integral parts of the signaling system are the Trainstops and Tripcocks. The Trainstop is securely attached to the right hand ends of the sleepers alongside the signal. Various electrical proving circuits pass through so that if there is a fault on this or the next trainstop ahead then it will remain up and the red aspect will stay alight giving a "dual aspect" if the section ahead is clear, The tripcock is part of the train. Fitted to the right hand leading positive shoebeam it is an emergency brake valve. In its normal position, pointing straight down, the valve is closed and the brake can be released by the driver. If the driver should overrun a signal at danger the arm strikes the trainstop, is knocked back opening the valve and applying the brake. To reset it, the driver must assist the train to stop and then pull a cord on the front exterior of the train. The brake applying opens a pressure switch cutting power to the motors.

Train is approaching a signal at Danger. The Trainstop head is raised and the Tripcock (to its left) is approaching the device.

The train has passed the signal at Danger. The Trainstop has done its job - the train has been tripped (note the Tripcock arm has been pushed back) and the Emergency Brake will have applied

In order to be certain that the trip arm is correctly to gauge testers are placed at various locations along each line. To check horizontal alignment passes between two upright posts and to pass the vertical test it must depress a spring loaded ramp. The driver's proof that all is in order is that the train is not tripped and that a dedicated light adjacent tot the platform starter goes out. This check is carried out at least once each journey.

                Tripcock Tester with the main components marked

Route indicators are the familiar line of white lights at junctions, Junction Route Indicators known as Harbour Lights. For shunting moves to two or more routes a matrix of individual lamps called a Theatre Route Indicator is used. When a move is set up "1" is used for the leftmost route, "2" for the second from left and so on.

Semi-Automatic Signal with Junction Indicator illuminated. In this example the signal is located at Hanger Lane Junction. The signal indicates that the route has been set towards Ealing Broadway.  If the Signal showed only a green aspect the route would have been set      towards North Ealing.

Shunting and subsidiary signals are white discs 15" inches in diameter and externally illuminated. Shunt signals have a horizontal red band rotate 45 degrees anti-clockwise to show "proceed". Where subsidiary signals are still in use they are again white but with two red lines across the middles and carry either "W" for a Warning Signal or "C" for a Calling On Signal. Subsidiaries are used when a second train is to enter an already occupied platform.

Shunt Signal at Danger, with Theatre-type Route Indicator above. When the signal is cleared the Route Indicator would illuminate showing the route number set.

                Calling On Signal - now redundant. This example is located on the eastbound approach to Parsons Green.

There are signals peculiar to LU. Two of these are "X" signals and "Draw-up" signals. The first of these is sited before special items such as floodgates and as the first signal after a Surface Stock Detector. This is a device sited on the approach to a tube stock loading gauge tunnel where surface stock might inadvertently gain access, such as the Piccadilly Line tunnel mouth at Baron's Court. They consist of a gantry with three "U" shaped mercury filled glass tubes suspended. If a surface stock train tries to pass the tubes will be broken and the signal will remain at danger.

Draw-up signals are miniature speed controlled three-aspect signals sited partway along platforms where there is a converging junction just ahead of the platform starter. When the route is set for a train to join the track in question an approaching train has to reduce speed to a predetermined level when the draw-up signal will clear to a yellow. The incoming train can then continue to the stopping mark.

Draw-Up Signal - speed of approaching train is being tested

            Train has reduced speed, the Draw-Up Signal clears to a yellow aspect as the signal to which it relates is at Danger

Draw-Up Signal shows a Green aspect, indicating that the signal to which it relates is also showing a Green aspect

Following the Moorgate incident in 1975 special measures were introduced at terminal stations, Firstly trains have their speed reduced by speed control signals on the approach. The traction current supply is the restricted to prevent the driver applying power to the motors. When entry to the platform is on an up grade limited current is available to overcome its effects. Sited along the Platform are "Policemen". These are trainstops without a signal and unless the train continues to reduce speed will remain raised and operate the trainstop operating the train.

Traction current rail gaps are marked by special signals which only illuminate when the current is off in the section ahead. "Rail Gap Indicators consist of three red lights in a white triangular plate marked as such. Their repeaters are similar but both lights and plates are yellow.

Signaling on both the Victoria and Central Lines is different as they are equipped for Automatic Train Operation.

Chiltern Railway class 165s permitted to operate over LU metals and those mainline stocks operating in single track tube tunnels are also fitted with tripcocks.


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