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Semi Automatic Signalling on London Underground

A description of the controlled or "semi-automatic" signalling on London Underground.  Links to other pages on signalling are listed in the column on the left.

Contents

Semi-Automatic Signals - Controlled Areas - Layout of Signals - Normal and Reverse - Signal Cabins and IMRs - Signal Identification - LU Signal Box and IMR Codes

Semi-Automatic Signals

Large areas of the London Underground network are controlled by automatic signals (described here) where no manual control is required.  Automatic signals operate entirely according to the passage of trains. 

London Underground also uses "semi-automatic" signals.  They operate in the same way as automatic signals in that they protect a section or block.  The difference is that, instead of returning to green automatically as soon as a train clears the block, they remain at "danger" until receiving a "proceed" instruction.  This can be provided either by a lever in a signal cabin or an instruction from a control room.  They are therefore designed to remain at danger (showing a red or stop indication) unless specially cleared by man or machine. 

Semi-automatic signals are used in areas where the routes for trains vary, i.e. junctions, reversing locations, depots and sidings.   All these areas include points. They are often referred to as "controlled areas".

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Controlled Areas

In an area where trains can take alternative routes, like a junction, entrance to a depot or at reversing points, the signals are controlled from a signal cabin or control room.  The signals around these areas will be semi-automatic signals.  A typical layout for a controlled area is shown below in Fig 2, where a reversing siding is provided at a station.

This layout, seen at a number of locations around London (Colindale, Tooting Broadway, Wood Green, Rayners Lane, Harrow-on-the-Hill, Marble Arch, to name but a few), allows trains to arrive in the northbound platform and for them to be sent into the siding or continue along the main route beyond signal A436.    Once a train is in the siding, the driver will change ends and, when the route is set and clear, the train can run out of the siding and into the southbound platform.

The layout shown above, where a reversing siding is provided at a station, is controlled from a signal cabin known as "NC".  All controlled areas are given code letters referring to the local room where the locking system for the signals is located.  All the semi-automatic signals controlled from this room are identified with these code letters.    All signals controlled from signal cabin NC will be lettered NC.

In addition to the code letters, each signal will have a number, which refers to the lever operating that signal.    Thus, signal NC2 will clear when lever number 3 in the signal cabin is reversed   - assuming the route it controls is set and clear. 

Note that points are also numbered according to the lever which operated them.  Normally, both ends of a crossover are given the same number because one lever controls both ends.  In Fig 1, the points are numbered 7A and 7B and 8A and 8B.  The letter A is always given to the end of the crossover nearest to the signal cabin.

Layout of Signals

In our diagram above, the approach to the platform in both directions is protected by three home signals.  This is common practice on the Underground - see Multi-Home Signals for explanation.  On the northbound (NB), the first two are automatics (A434A and A434B) but the last one (NC 1) is a "semi".  This is because it is necessary to control the approach to the points at the far end of the platform.  A train might be held at NC 1 if, for example, the points were being changed after a train in front had entered the siding.

The starting signal, NC 2 has a shunt signal below it, NC 3.  If the train is required to proceed NB, NC 2 will show a green and the shunt signal NC 3 will remain showing a stop aspect.  If the train is required to go into the siding, NC 2 will show red but NC 3 will clear, showing the red bar at 45 degrees.

On the SB side, the three home signals (NC 12A, NC 12B and NC 12C) are all controlled from the same lever.  If the points are set and the route is clear into the station, all three signals will clear when the lever is reversed.

For the different types of signals and their meanings, see Types of Signals.   There are lots of variations of signal layouts and operation but the principles are the same throughout the system.

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Normal and Reverse

Semi-automatic signals are traditionally controlled by levers.  Signal levers have two positions, "normal" and "reverse".  In the normal position, the signal will remain red or points will lie in the direction of the main route.  In the reverse position, the signal will clear or the points will be set for the diverging route.   When a lever or points are returned to the normal position, they are said to have been "normalised".

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Signal Cabins and IMRs

Until the late 1950s, all controlled areas were operated by signal cabins, employing one or more staff to set up routes and record the passage of trains.  Since then, there has been a gradual move to centralised control rooms, starting first with the Northern Line and then spreading to most of the rest of the system.  There are now very few signal cabins left on London Underground.  They have been replaced by Interlocking Machine Rooms (IMRs) and the modern equivalents, Signal Equipment Rooms (SERs).  Both are local sites containing all the equipment necessary to control the area but they are remotely controlled from a control centre.  The Bakerloo line, for example, has its control centre at Baker Street and the Jubilee Line's is at Neasden.

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Signal Identification

Automatic signals are identified by a number preceded by the letter A.  Semi-automatic signals, i.e. those controlled from a signal cabin or interlocking machine room (IMR) are identified by code letters and a number pertaining to the route controlled by or the lever operating that signal. 

For sample photos, Click on image for the link to a set in full size views. 

A list of the codes in use on London Underground appears below.

Annn - automatic signal.  These are numbered like houses along a street, odd numbers on one side and even numbers on the other side.
Rnnn - repeater for automatic signal.  It uses the same number as the signal it repeats but replacing the A prefix with an R.
Annnn - automatic signal with approach control facility - these are found near the termini, e.g. A1004 at Mill Hill East makes sure the train's speed is reduced.
Xnnn - automatic signal that can be held at danger if required and which must be treated by drivers as a semi-automatic signal.
MW(or any other pairs of letters)nn - semi-automatic signal, which must not be passed at danger without proper authority as points may be involved. The code may be one or two letters according to the area and the code refers to the signal's IMR or signal box.
MW(or any other pairs of letters)n00 - draw up/pull up signal (sometime referred to as a permissive signal) which is linked to the signal ahead and makes sure speed is brought down to a given level. (often 10mph)
MW(or any other pairs of letters)X.nnn - automatic signal which must be treated as a semi-auto for passing at danger rules. Normally found on the approaches to semi-automatic areas where approach control is likely to lock up the route.
MW(or any other pairs of letters).nn.nn - rather old fashioned way of numbering a signal with two routes.  Examples would be at Harrow-on-the-Hill and Queen's Park BB34/331 etc, which is an extension of the concept.
MWnn/1 - hard to describe.  Basically, a signal that's ''split'' into different parts. You pull one lever on the panel and whichever signal is appropriate for a route will clear.
LEY A4513 - Central line signalling example, the three letter area code denotes an interlocking running from an SER, in this case Leyton.  Some signals may be block boards.

Information supplied by 3518+3227 in uk.transport.london 10 September 2000.

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LU Signal Box and IMR (Interlocking Machine Room) Codes

Here are the codes for all LUL lines at present, plus some past codes - most codes were just single letters originally and later began to be prefixed with another letter (usually the line initial) for more precise recognition - all codes in this list have been verified to be correct:

This information supplied by Raggajohn, uk.transport.london 10 July 2000 and corrected by Tubeprune.

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