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Semi Automatic Signalling Page

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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.


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.

sig semi1.gif (12434 bytes)

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. 

Starter_at_EB_WP_55_small.jpg (3204 bytes)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 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:

Bakerloo Line
Queens Park - BB
Paddington - BD
Baker Street - BM
Piccadilly Circus - BP
Lambeth North - BR
Elephant - BS

Victoria Line
Brixton - VE
Victoria - VF
Warren Street - VG
Kings Cross - VH
Highbury - VJ
Finsbury Park - VK
Seven Sisters - VL
Northumberland Park
Depot - VN
Walthamstow Ctrl - VP

Circle Line/Met codes 1904-1950's
A - Praed Street (later Edgware Road)
B - Baker Street
C - King's Cross
D - Farringdon
E - Barbican
F - Moorgate
G - Liverpool Street
H - Aldgate
Circle Line/H&C Codes 1950's-Present
OB - Aldgate/Aldgate East
OD - Liverpool Street
OE - Moorgate
OG - Barbican
OH - Farringdon
OJ - Kings Cross
MB - Baker Street
OP - Edgware Road
OR - Paddington Suburban
OS - Westbourne Park
OU - Ladbroke Grove
OZ - Hammersmith
Metropolitan Main Line (original codes)
B - Baker Street
D - Finchley Road
E - Willesden Green
F - Neasden
G - Wembley Park
H - Harrow South
J - Harrow Main
N - Harrow North
P - Rayners Lane
R - Eastcote
V - Hillingdon
W - Uxbridge
Ruislip cannot be confirmed in this series.
Piccadilly Line - Original Codes (Present Codes)
Codes west of Barons Court are under the District 'W-' series.

A - Hammersmith Pic (WD from 1932)
B - Down Street/Hyde Park Cnr (now PB)
C - Covent Garden
D - Holborn (now PD)
E - York Road
F - Finsbury Park (now VK)
G - Wood Green (now PG)
J - Arnos Grove (now PJ)
K - Oakwood (now PK)
M - Cockfosters (now PM)
Central Line (Pre-resignalling)
West Ruislip CS
Ruislip Gardens CS (subsiduary to West Ruislip)
Northolt - CP
Greenford - CL
Ealing Bdy - WP (was L)
North Acton - CJ
White City - CG (was G)
Shepherd's Bush - CF (was F)
Queensway - CE (was E)
Marble Arch - CD (was D)
Holborn - CC (was C)
Bank - was B
Liverpool St - LB (was A)
Bethnal Green  - LC
Stratford - LD
Leyton - LE
Leytonstone - LF
Snaresbrook - LG
South Woodford - LH
Woodford - LJ
Grange Hill subsiduary box to Hainault LL (unoffically LK)
Hainault LL
Fairlop - LM
Barkingside - LN
Newbury Park - LP
Loughton - LT
Debden - LU
Theydon Bois - LV
Epping - LW
North Weald (until 1976) - LX
Ongar - LX
District/Piccadilly codes west of Earls Court begun with W, those east beginning with E - those principles remain more or less today.

WA - Earls Court West (now EC)
WB - West Ken East/Olympia
WC - West Ken West
WD - Hammersmith
WE - West Brompton
WF - Parsons Green
WG - Putney Bridge

(WH) - Junction with LWSR at ravencscourt Park?
WK - Turnham Green

WL - Acton Town
WM - Ealing Common
(WN - Hanger Lane Jcn (WM from 1925)?)
WP - Ealing Broadway

WQ - South Acton
WR - Northfields
(WS - Boston Manor (until 1932)?)
WT - Hounslow Central
WU - North Ealing (closed)
WV - South Harrow
WW - Hatton Cross (was Sudbury Hill)
WX Hounslow West (until 1975)
WY - Heathrow

EA - Earls Court East (now EC)
EB - High St Ken (now ED)
EC - Earls Court (was Cromwell Rd)
ED - High St Ken (present)
EE - Gloucester Road
EF - South Kensington
EG - St James Park (Closed)
EH - Embankment
EJ - Tower Hill (was mansion House)
EK - Minories Junction
(Now OB)
EL - Aldgate East (Now OB)
(EM - St Marys Jc until part of EN?)
EN - Whitechapel
EP - Bow Road (closed)

ER - Surrey Quays
ET - New Cross area

B - Shoreditch/Whitechapel (ELL)
OB - Aldgate East

FB - Bromley
FC - Plaistow
FE - East Ham
FF - Barking
FG - Dagenham East
FJ - Hornchurch
FM - Upminster
Northern Line Original Series
A - Kennington (when all autos prefixed S)
B - Kennington (present)
C - Charing Cross
D - Mornington Crescent (until 1924)
E - Camden Town/Mornington Cres (from 1924)
F - Hampstead
G - Golders Green
J - Euston (+ Kings Cross from 1967)
K - Angel
L - Kings Cross (loop only from 1967)
M - Moorgate
P - London Bridge
U - Stockwell
T - Clapham Common
W - Tooting Bdy
Y - Morden
Z - Morden Depot (not official)

1920's extension codes
AA - Brent Cross
AB - Hendon Central
AC - Colindale
AE - Edgware

1930's extension codes
NC - Drayton Park/Finsbury Park
ND - Moorgate (Northern City)
NH - Park Junction (now Highgate Depot)
NN - Archway
NP - East Finchley
NQ - Finchley Central/Mill Hill
NS - Woodside Park
NT - Totteridge
NU - High Barnet
NV - Mill Hill East (now NQ)
Metropolitan Main Line Present Series
MB - Baker Street
MD - Finchley Road (+W Hampstead until 1985, now JD)
ME - Willesden Green (JE from 1985)
MF - Neasden (now MM)
MG - Wembley Park
MH - Neasden Depot North End (closed)
MK - Stanmore/Canons Park (JL from 1985)
ML - Swiss Cottage
MM - Neasden (present)
MP - Rayners Lane
MR - Eastcote
MU - Ruislip
MV - Hillingdon
MW - Uxbridge

JB - Harrow
JD - Finchley Road/W Hampstead Jubilee (from 1985)
JE - Willesden Green (from 1985)
JF - Northwood (was Neasden)
JG - Wembley Park Jubilee (1985)
JJ - Watford South
JL - Stanmore (from 1985)
JP - Watford/Rickmansworth
JT - Chalfont/Chesham
JW - Amersham
Jubilee Line
Stanmore - JL (MK until 1985)
Canons Park - MK
Wembley - JG (MG until 1985)
Neasden - JF (MF until 1985)
Willesden Green - JE (ME until 1985)
West Hampstead/Finchley Rd - JD (MD until 1985)
Baker Street - BM
Green Park - TF
Charing Cross - TG
Waterloo - TH
London Bridge - TJ
Canada Water - TK
Canary Wharf - TL
North Greenwich - TM
Southwark - TP
Bermondsey - TR
West Ham - TU
Stratford Market Depot - TY
Stratford - TW

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

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