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Underground Dictionary and Glossary

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London Underground Glossary

London Underground has its own language.   Here is the start of a list of some the most commonly used terms and expressions.   More will be added from time to time.


A Stock - The original A Stock was built for the District in 1903 as the test for the electrification of the line and which were used on the Ealing and South Harrow line.  The present A Stock is the standard type used on the Metropolitan Line.

AFC - Automatic Fare Collection

Alps - The nickname given to the sidings at the north east corner of Ealing Common depot where the tracks are at a higher level than the rest of the depot.

Approach Controlled Signal - a signal on London Underground which remains red, even if the route it protects is set and clear, until the train has occupied a track circuit close to the signal.  This form of control is used to force drivers to reduce speed if required because of a curve or other restriction.

Arrestor - A device located in rear of buffer stops for absorbing a limited amount of the energy of a train which overruns the usual stopping point at a terminus or dead end.  The maximum speed for a tube train reaching an arrestor cannot be more than about 20 km/h without the train sustaining some damage.

ATO - Automatic Train Operation, where the train speed is controlled by a combination of commands transmitted from the track or trackside and the operator (formerly the driver) only has to start the train and operate the doors.  Used on the Victoria Line throughout and on the Central Line except part of the Ealing branch.   For more on Victoria Line ATO, click here.

ATP - Automatic Train Protection, where the train is prevented, automatically, from passing a stop signal.   There are various ways of doing this mechanically and electronically.  London uses a trainstop system on most of its lines and electronic systems on the Victoria and Central Lines.

Automatic Coupler - See Wedgelock Coupler

Automatic Signal - A signal which normally shows a green aspect.  The signal is not provided with manual control and responds to the condition of the track circuits controlling it.  The signal will go to danger when a train passes.   It will automatically revert to a green aspect when it is safe for trains to pass or a lunar white aspect when it is safe for automatic trains to pass. See London Underground Signalling.


B Stock - The original wooden stock introduced on the District for its main line electrification in 1905.  It was developed from the trial A Stock and originally comprised 420 cars formed into 7-car trains.  Some of it lasted into the 1940s.

Bakerloo - The line which was originally the Baker Street and Waterloo Railway and which now operates between Elephant & Castle and Harrow & Wealdstone.  The route is coloured brown on the Underground map.  

Banjo - Shunt signal (slang).  See also Dolly and Signalling.

Block Joint - An insulated joint fitted in a rail to divide two adjacent signalling sections by ensuring that the track circuits of the sections are kept separate.  See also Signalling.

Bostwick gates - Lattice type gates that are pulled across an opening such as a station entrance or lift.  Named after the company which originally made them.

Branch, The - Slang term for the Metropolitan Line north of Baker Street, dating from the time when the main line was the section between Paddington and Farringdon and a single track branch was opened to St Johns Wood.  See also "Wood Line".

BRS Plate - British Reference System. It's used to measure the distance from one station to the next.  The distance (usually every 100 metres) is marked by a BRS plate. For example, Walthamstow Central Southbound on the Victoria Line is identified as V027 and, if you walk 200m along the tunnel towards Blackhorse Road you'll see a BRS plate on a sleeper with "V027 200 M" printed on it.  It's mainly used to identify exactly where a particular spot on track is.

A much better system is the chainage system, which is referenced from one spot (on LUL it's Ongar), and everything is measured from there.  It's so much easier to calculate distances between 2 points using chainages (called "Ongars" on LUL).  Therefore, LUL uses BRS, while the whole rest of the world uses chainages.  Information in reply from "Jon" to a query on, 31 March 2002.

Buffer Stops - A device at the end of a section of track to act as a marker and to allow a train to rest against them if required.  Normally also provided with some sort of arrestor device like sand, gravel or mechanical retarder.

Busline - Term used by LU to refer to a power cable running the length of a train or unit, which supplies all high voltage systems.  Buslines were forbidden on the tube lines and, in the days when trains used 600 volt circuits for most control as well as power circuits, elaborate precautions were taken to ensure that power connections between cars never carried the full traction power current.  The term originated from the word "omnibus" (Latin for all), shortened to 'bus.

Button, Drop the - Phrase used to indicate that the driver has released the vigilance device ("deadman") causing an emergency brake application.  Synonymous with "dropping the deadman".  The term "button" refers to older Underground trains which were equipped with a button, in the centre of the master controller handle, which operated the vigilance device.  This type of handle is now only seen on some LU battery locos.


C Stock - The original C Stock was a type of steel bodied stock built for the District in 1910, some cars of which survived to the late 1950s.  The later type is the C Stock, now the standard type used on the Circle and Hammersmith Lines and on the Wimbledon - Edgware Road service of the District.

CWR - Continuous welded rail - Stretches of track where sections of rails are welded to make very long sections. It eliminates fish-plated joints, where most rail defects occur.

Calling-On Light - An orange light provided on cab ends of trains which, when illuminated, signals to the driver of a following train to draw up to provide assistance, usually when a "push out" is required by a defective train.  The calling-on light was first introduced on the 1967 Tube Stock because there was no guard on the train to perform the calling-on function.  The 1967 Tube Stock was the first train on the Underground to be designed for one-person operation.  Previously, trains carried a crew of two, a driver and a guard.

Calling-On Signal - A type of shunt signal used to permit trains to proceed into occupied tracks, usually for the purpose of coupling to another train or unit.  Not now regularly required and quite rare.  Samples survive at parsons Green (District) and Watford (Met.).  Up until the late 1960s all lines were equipped with them.

Cant - A standard railway term describing the difference in level between the two rails of a track.   The outer rail of a curve is higher than the inner rail to counteract the centrifugal force of a train travelling around the curve.  The amount of difference in height of the two rails is the cant. The maximum is 150mm.

Car - London Underground rail vehicle.  Replaces the usual British "carriage" or "coach".  Originated in the US because electric multiple-unit traction was imported from there into the UK.

Car countup and station limit markers - Indication plates positioned along the track leading from a station platform, which carry a white reflective number on a blue background.  They provide the Train Operator with information as to the train’s position in relation to the platform following operation of the passenger emergency alarm/brake. They count up the number of cars' distance from the platform (up to eight).

Car Examiner - The now largely defunct name for train technician.  Also known colloquially as a "fitter".

Category ‘A’ platform - A platform where the Train Operator needs assistance from other staff to see the full length of the train when OPO equipment has failed.  If OPO equipment becomes defective the platform must be staffed or trains non-stopped.

Category ‘B’ platform - A platform where the Train Operator does not need assistance from other staff to see the full length of the train when OPO equipment has failed. 

Centurion Manager - An operating manager responsible for overseeing around 100-150 staff.

Cess - The space alongside the railway between the ballast and a cable run or other boundary.

Chair Lock - A design of point machine introduced on to London Underground in the early 1970s.

Check Rail - A length of rail (on any railway, not just LU) which is fitted inside the running rails to assist in preventing the train's wheel flange moving outside its normal gauge at points and crossings and on sharp curves. It should not normally come into contact with the train's wheels.

Circle Line - Much maligned route (yellow on the map) which forms a roughly circular (naturally) route around the West End and City areas of London.   It was originally built 1863 - 1888 to connect the main line stations around the edge of the then most built-up areas of London.  The service is particularly difficult to operate because of numerous flat junctions which connect the line to various routes on the Metropolitan and District Lines, with which it shares tracks.

Clamp Lock - A design of point machine introduced into London Underground in the 1970s. 

Clip & Scotch - A means of securing points which have failed to operate or indicate that they have locked in the required position.  The clip is placed under the closed point blade and the stock rail to clamp the two together.  The scotch is pushed into the gap between the open point blade and the other stock rail.

Collar - Any device provided for use as a reminder that a lever, push-button, or switch must not be operated, usually in a signal control room.

Conductor Rail Ramps - Ramps (picture here) fitted at the ends of current rails that allow the train's shoes to slide on and off smoothly.

Controlled area - An area where the movement of trains is controlled by or affects the operation of semi-automatic signals.

CRID - Current Rail Indicator Device, used to show if current is on.  Not to be confused with Rail Gap Indicator which shows where current is off.  CRIDs have small orange lights and are located at the end of some station platforms, just inside the tunnel.

Current rails - The two additional rails provided on London Underground track to act as conductor rails for the traction power supply.  The trains collect current through cast iron "shoes" provided on the bogies. 

Customer - A person who purchases from a shop.  The same term is now applied universally by London Underground to its passengers, even though the majority of them prefer to be called passengers and even write letters to the papers to say so.

CWR - Continuous welded rail, a track type used by railways all over the world.   It eliminates plated joints, where most rail defects occur.


D Stock - The original D Stock was a type of steel bodied stock built for the District in 1912, some cars of which survived to the late 1950s.  It was indistinguishable from the District's C Stock of 1910 (see above).  The later type is the D78 stock, now the standard type used on the District.

Deadman - A shortened version of "Deadman's Handle", the driver's vigilance device consisting of a sprung power controller handle which, when released, causes the train brakes to give an emergency application and to switch off traction power to the motors.   London Underground has always used controller handles for the vigilance device rather than the pedals favoured by many other railways.

Detonator - A warning device, much beloved of railways, secured to a running rail, which creates a loud bang when run over by a train or mechanised vehicle.  Used a a warning device to alert drivers that the train must be stopped.

District Line - The sub-surface line (green on the map) covering the southern half of the Circle and branches to the east and west of London.  Difficult to operate, like the Circle, because of the flat junctions and variety of routes.

Dolly - Shunt signal (slang).  See also Banjo.  See London Underground Signalling.

Donkey Dick - (Slang) A ready to start plunger provided in a siding so a driver can alert the signalman when he is ready to go.

Dreadnought - A nickname given to batches of Metropolitan Railway, locomotive-hauled coaches which first appeared in 1910.  They were a considerable improvement on earlier coaches and were likened to the new class of battleships then being built for the British Royal Navy.   Photo here.

Dual Aspect - A signal showing both proceed and stop aspects at the same time.  This is normally due to the trainstop failing to lower when the signal clears.  The proceed aspect indicates that the line ahead is clear but the stop aspect indicates that the trainstop has failed to lower when the signal cleared.  See also Trainstop.  

Dump Valve - Brake valve used on 1973 and D Stocks to dispose of brake cylinder air for remote brake isolation purposes.  The dump valves on 1973 Stock were originally linked to a wheelslide protection system but it was taken off some years ago since it was found to aggravate the occurrence of wheel flats.


E Stock - Type of steel bodied stock built for the District in 1914, some cars of which survived to the late 1950s.  It was virtually identical to the C and D Stocks of 1910-12 except that it had an elliptical roof in place of the usual District clerestory roof of the period.

ECEB - Electrical Control of Emergency Braking -The system adopted on the 1973 Tube Stock and all subsequent trains, where the on-train safety braking system is electrically monitored instead of the compressed air brake pipe used on earlier stocks.  A round-the-train circuit is used to monitor the completeness of the train, brake functions and safety devices like the tripcock and deadman.

Electrical Control Room - A room provided for the remote supervision of electrical substations.

Emergency release - A signalling control facility used to release the electrical locking on signal and/or points levers or shafts.

Engineering hours - The period between the published time or actual time, if later, traction current is switched off and the published time or amended time, if earlier, traction current is switched on.

ERU - Emergency Response Unit. Used to be known as "the Breakdown Gang".


F Stock - 100 cars of all-steel stock built for the District in 1920 (Photo).  After the second world war they were transferred to the Metropolitan Line and were replaced by the present A Stock in the early 1960s.

Fish & Parcels - Old slang name for the District Line, since it runs over main line routes and was regularly alleged to be subject to delays from other railways.  In the days when it shared the two tracks between Bow Road and Upminster with the London, Tilbury and Southend route, District trains were regularly delayed by freight trains.

Four foot - The space between the two running rails of a track.  Originally from "four foot eight and a half inches", the gauge.


G Stock - Type of clerestory roofed stock built for the District in 1923 and subsequently absorbed into the Q Stock.  All cars of this batch were originally motor cars.

Gap (in current rails) - A section of track where the current rails are discontinued because of points or a sub-station feed. 

Gap Jumper - Portable connecting leads often provided at locations where there are a number of current rail gaps due to a complex junction.  The leads are provided with a plug at one end to allow connection with a receptacle box on a train, and with contact shoes at the other end to allow them to be placed on the current rails.  If a train becomes "gapped", the gap jumper is used to connect a car on the train to nearby current rails.  They are awkward to use and need care to restart the train without dragging the leads or running over them.  Sometimes called gap leads.

Gapped - Getting "gapped" occurs when a train stops with all its shoes off the current rails and therefore without power to restart.  Very embarrassing.

Gap, Mind the - Announcement much beloved of London Underground management and offered at any opportunity where the edge of the platform is more than a few millimetres distant from the sill plate at train doorways.   The expression had been used for many years at locations like Waterloo (Bakerloo), where a large gap exists on a sharp curve.  It was introduced at many more locations in the early 1990s following the reduction of staff on platforms and a subsequent rise in reported accidents to passengers.  The expression is now the butt of many bad jokes in London and is meaningless to passengers except as an irritant.

Guard - The person formerly employed to operate the doors on an Underground train, look after passengers, ensure right-time departure from stations, assist the driver in emergencies and provide rear protection for the train if required in emergency.  All trains are now one-person-operated (OPO) and the guard's job is redundant.


H Stock - A batch of former B Stock cars dating from 1905 and rebuilt in the mid 1920s for the District.

Handing - The identification system used by LU to ensure that cars are coupled correctly and to identify the position of wheels and equipment.  More detail.

Headwall plunger - Equipment provided (usually only at tunnel stations where there is a sub-station gap) to switch off traction current in an emergency.

Headwall tunnel telephone - A telephone in a sealed box located on a platform headwall which is connected directly to the tunnel telephone line system.  It is used to switch off traction current in an emergency.

Headway Post or block marker board - Where a train in automatic working will be stopped if the section ahead is occupied.   The train will automatically restart when the section ahead clears.  It works similarly to an automatic signal on Automatic Train Operating line(s) but does not display signal aspect(s).   See also Victoria Line ATO.


IMR - Interlocking Machine Room, where local signalling control and interlocking equipment is located.   The modern equivalent is the SER or Signalling Equipment Room.


Jubilee Line - Tube line originally opened in 1977 when it took over the Stanmore branch of the Bakerloo Line combined with a new route from Baker Street to Charing Cross via Bond Street.  An extension to Stratford from Green Park was opened in November and the line to Charing Cross was deprived of its passenger service.  Grey on the map.   See also photos of the extension stations.

Jumper - Multi-core cable used to connect electrical circuits between cars.


K Stock - Type of clerestory roofed stock built for the District in 1927 and subsequently absorbed into the Q Stock.  All cars of this type were originally motor cars.

King Lever - A device in a signal cabin used to make semi-automatic signals operate automatically.  Now sometimes called "automatic working switch", since levers are not so common.

Klondyke Sidings - The name given to the sidings at Neasden Depot, which are laid between the main sheds and the main Metropolitan Line.  The sidings are connected to the fans at both ends of Neasden Yard and are used for shunting movements as well as for stabling trains.   The origin of the name is unknown but it was suggested that it arose because they were first provided at the time of the Klondyke gold rush of 1897-8.  Also suggested by "Seek Assistance" that the name arose because the trains used in the peak hours were parked there during the middle of the day and at night and they were renowned among train cleaners for being a treasure trove of coins which had fallen down behind the seat cushions etc.

Khyber Pass - Slang referring to the small gateline section leading out of Kings Cross tube station ticket hall facing towards the subsurface ticket hall.


L Stock - Type of clerestory roofed stock built for the District in 1931 for the Upminster extension and subsequently absorbed into the Q Stock fleet.

Leads - Portable cables used to assist trains which have been "gapped" i.e stalled across tracks where there are gaps in the current rails.  The leads consist of a pair of cables, one positive and one negative with a plug at one end and "shoes" at the other end.  The plug is fitted into a socket on the train while the shoes are placed on the nearest available current rails.  The power thus made available to the train is used to move it far enough to allow its own shoes to make contact with the current rails.  The leads are then removed.  Leads are usually stored in boxes at locations around the system where long gaps in current rails are known to give problems to trains.

Line Controller - A person located in a Line Control Office to co-ordinate the day to day working of the railway.

Line diagram - A visual display in a signal box, Regulating Room, Signalling Control Centre, Service Control Centre, or Line Control Office showing the state of track circuits, signal aspects, the movement of trains, the position of points at some locations and train descriptions.

LWR - Long Welded Rail, a track type like CWR.  Information in reply from "JDikseun" to a query on, 31 March 2002.

Lookout - A person specially trained and used to warn staff who might be exposed to danger from moving trains or vehicles.


M Stock - Type of clerestory roofed stock, based on District rolling stock designs, built for the Hammersmith & City Line in 1935 and subsequently absorbed into the Q Stock fleet.

Main Line - The pipe carrying the compressed air supply for the train, which is connected between cars by hoses or automatic couplers.

Main Reservoir - A compressed air receiver mounted under each car (on most stocks) to serve as storage for compressed air. Air is usually supplied at between 70 to 120 lb/in2.

Mate - The name used by a trainman when referring to his colleague on a train in the days of two-person operation.

Mechanical Governor -  Device used on the Victoria Line 1967 Tube Stock to restrict the speed of the train to 25 mph under certain conditions.  It comprises a rotating device which uses centrifugal force to open a contact in a circuit at a set speed (25 mph for the Victoria Line ATO) to cause a control action.  The same device is used in older lifts to prevent overspeed.  For an explanation of its use on the Victoria Line, see Victoria Line ATO.

Milking the tubes - A trick used by dishonest booking clerks before computerisation when individual tickets were taken from the middle of a sequentially numbered stack instead of from the bottom.   This prevented accurate accounting at the end of the day and covered for cash which was illegally removed.  The tickets were stored in vertical tubes and were designed to be removed from the bottom of the tube as required for selling to passengers.  Accounting was done by noting the number of the first and last tickets sold of each value and multiplying the difference by the face value.

Motor Alternator - A machine provided on trains, built before the 1990s on London Underground, to supply low voltage auxiliary systems like car lighting, battery charging and control circuits.   It consists of a 630 volt DC motor powered off the traction supply, which drives an alternator.  This alternator produces AC for lighting and for conversion to DC for battery charging.  the voltage levels vary from stock to stock.  The modern equivalent is called an auxiliary converter.

Motor Generator - A machine which used to be provided on trains to supply low voltage auxiliary systems like car lighting, battery charging and control circuits.  It consists of a 630 volt DC motor powered off the traction supply, which drives a DC generator.  On London Underground, the generator produced 50 volts DC for lighting, control circuits and battery charging.  It was superseded by the Motor Alternator.  The modern equivalent is called an auxiliary converter.

Motorman - The title which used to be used by London Underground for a driver.  It fell into disuse because of the introduction of the title "Train Operator" which was used for trainmen employed on trains equipped with ATO.  Drivers were always promoted from guards who, in the days of two-person operation, used to operate the doors and act as the second man in emergencies and for train protection.  Now, all drivers are officially called Train Operators.

Multi-aspect signalling - An area where a stop signal can show a single yellow caution, double yellow caution or a green aspect provided the section ahead is clear.  Some signals are 3-aspect, showing only a single yellow in addition to the green and some are 4-aspect, with the double yellow.   The signal will revert to danger when a train passes. 

Mutual - The shortened version of "a mutual changeover", the swapping of duties between station staff or trainmen.  By this process, two people could agree to swap duties and then inform the supervisor by mutually signing a "changeover slip".


N Stock - Type of clerestory roofed stock built for the District in 1935 and subsequently absorbed into the Q Stock.

Network Control Centre - (NCC) the headquarters control room for the Underground system located at 55 Broadway, London.  Each line has its own control organisation but the Underground network is monitored from the NCC.  The role is one of co-ordination between lines and with outside organisations like Railtrack, the police and fire services.


O Stock - Type of train, now extinct, which was first introduced in 1937 for use on the Hammersmith and Circle Lines.  It had an unusual flared body profile, typical of the innovative design solutions adopted by the Underground in the 1930s but sadly lacking these days.   The flare was intended to cover the gap between the train and the platform.   During its career, the stock worked over all the Underground's sub surface lines at one time or another.  It was unusual at the time in having the guard's controls in the driver's cab, as it was the intention to convert them to one-person operation.  This was never done in the lifetime of the stock.  The O Stock was soon absorbed into the next batch of cars known as the P Stock and they operated together.   The O Stock had an unusual type of traction control known as the Metadyne, which was intended to allow regenerative braking.  The system gave much trouble in service and was converted to PCM control over a 5-year period in the late 1950s and early 1960s. Photo here.

Off - of a signal, meaning that the signal is showing a clear or proceed aspect.

On - of a signal, meaning that the signal is showing a stop or caution aspect.

On the cushions - When a train operator's 'next pickup' requires that he rides back on a train to his home depot, he is said to be going "on the cushions" or "on the covers".   Similarly, if a train driver has only one train left to pick up in his duty, that train is cancelled and he is nonetheless told to go to his next pickup, this is code for 'go home' which his manager cannot directly say since he is being paid to the end of his duty even though there is nothing else for him to do. 

On-track plant - Self-propelled rail mounted mechanical equipment used during the construction and maintenance of railway infrastructure.

OPO - One Person Operation; the standard train operating regime used on London Underground.   Conversion from two-person crews began in 1984 and was finally completed in 2000.

Out of Gauge Rails - Rails which do not retain the required distance between them.  Rails on LU should be 1432mm apart (or 1435mm on flat curves, 1438mm on tight curves).  If they're not, the rails are out of gauge.  Information in reply from "JDikseun" to a query on, 31 March 2002.

Overhead Leads - The standard LU shore supply system for trains in depots where there are no traction current rails.  A simple, two-rail electrified track is suspended from the shed roof and a trolley slides along the rails so it can be positioned anywhere along the road(s) it supplies.  A pair of insulated leads (positive and negative) hang from the trolley to floor level and are equipped with a plug.  The plug is inserted into a socket provided on certain cars of the train to provide power.  The socket is called a receptacle.  On stocks built in the 1990s, this socket is equipped with an isolating switch to prevent collector shoes from being made live when the overhead lead is inserted.   More details and photos here.


P Stock - Type of train, now extinct, which was first introduced in 1938 for use on the Metropolitan and District Lines.  It was very similar to the O Stock except that it had the guards controls in the passenger saloon, as on other Underground stocks.  During its career, it worked over all the Underground's sub surface lines at one time or another. It was quickly absorbed with the O Stock and was soon formed into mixed trains of O/P Stock.   It had an unusual type of traction control known as the Metadyne, which was intended to allow regenerative braking.  The system gave much trouble in service and was converted to PCM control over a 5-year period in the late 1950s and early 1960s.

Passenger Emergency Alarm (PEA) - A system provided on trains for passengers to communicate to the Train Operator that an emergency situation is present.

Passenger Emergency Alarm Brake - A system on a train that, when operated by activation of a passenger emergency handle, will alert the Train Operator to a problem and will automatically apply the brakes.  It can be over-ridden by operating a brake release floor switch.   This is to allow a train to continue to the next station where assistance can be provided.

PCM - Pneumatic Camshaft Mechanism - The name used by LU for the traction equipment used on its trains which are equipped with pneumatically operated power resistance controllers, i.e. any stock built before 1992.  The name is derived from the US system known as PCC control, in turn derived from the President's Conference Committee of the American Institute of Electrical Engineers.

Phantom signal - A semi-automatic stop signal in an ATP area (Central Line). It does not display a colour light aspect but a target speed is displayed in the Train Operator’s cab.  Its location is identified by a sign showing its signal code.

Pictures - The name of the siding at the eastern end of Northfields Depot, which was used for shunting and which was close to the local Cinema.

Pipe - see Rounder.

Plunger (1) - A button mounted outside the train which the driver presses to indicate to the train regulator (or programme machine, or signal computer etc.) that he
is ready to start from a siding or platform.  Information in reply from "JDikseun" to a query on, 31 March 2002.

Plunger (2) - A small piston in the pump of a track or flange lubricator.  The train wheels depress the plunger as they pass, pumping grease onto the spreader bar.  The flanges of the wheels pick up the grease from the spreader bar (sometimes, when you're lucky) and then (sometimes, when you're really lucky) deposits the grease onto the gauge corner of the rails.  The grease reduces wear of the rails (so extending the life of the rails to reduce costs) and cuts down the noise called flanging.  You can always tell when the greaser (lubricator) isn't working - you hear the high-pitched squeal.  Information in reply from "JDikseun" to a query on, 31 March 2002.

PNR - Physical Needs Relief - The use of spare staff to allow a person to leave his post or duty to use the lavatory.   The facility is much abused by trainmen to obtain a break from the tedium of train operation in tunnels, even if they don't need it for natural causes.   Sometimes referred to as a "physical" or a "sh*t relief".  The spare person takes over part of the duty of the person requiring the "PNR", including part of the trip within a train operator's duty.

Porter's button - A button provided at the end of each car, normally a black rubber disc but can be white or grey, used for closing the train doors on that car only.

Position light shunt signal - A shunt signal that will show two red lights horizontally when at danger.  It shows one white light when clear.  A newer type of shunt signal seen on LU since the resignalling of Neasden Depot in the 1980s.

Possession - A section of track which has been relinquished by the operating railway to the engineer.  Control is in the charge of a Possession Master.  Unauthorised train movements into the section are prevented by special protection arrangements.

Possession Master - A person specially trained and certificated to take control of and give up a possession, in order to carry out engineering and similar work.

Protection - Procedures to make sure staff on the track are not endangered by a moving train or mechanised vehicle.

Protection key switch - A signal control device used to maintain semi-automatic signals at danger to provide protection for staff entering tunnels during traffic hours.

Protection Master - A person certificated by LUL to provide protection for himself and others on or about the track.

Punters - Slang term for passengers.

Push Out - The rescue of a failed train by the following train, which is used to provide power by pushing the defective train to the depot.  The process invariably causes a long delay because the coupling is slow (largely due to unfamiliarity of the crew with the coupling system, since they don't get to use it very often) and the two trains must be detrained one after another at the next station.  Speed is severely restricted and the double length train causes track circuits to respond in unusual ways - normally locking signals at danger, causing further delays.  Not to be recommended.


Q Stock - Type of old rolling stock used on the District until the early 1970s. It was formed from a mixture of older stock and some new vehicles introduced in 1938.  The mixture of the old and new cars gave the stock a disorganised appearance.   Photo here.


'R' board - A sign displayed at a station when there is defective trackside radio equipment ahead.  It is red or white and has a black "R" with a white cross.

R Stock - District Line rolling stock introduced in batches from 1947 to 1959.  The 1949 batch (known as R49 Stock) was the first London Underground stock to be constructed with aluminium bodies and some cars were delivered with unpainted bodies.  This experiment was so successful that unpainted bodies were universally adopted by LU until the 1992 Tube Stock.  Photo here.

Rail anchor - A device which when applied to the foot of the running rail and in contact with the chair resists the longitudinal movement of the rail. The name was originally used (and is still) to mean a train anchor.  See also train anchor

Rail Gap Indicator (RGI) - Explanation and photo.

Railing - The act of moving a train in a shed without current rails so that the leading car is standing on current rails outside the shed.  Photo here.

Ramp, current rail - Sloping end of a current rail designed to allow the collector shoes to ride onto and off the rails without damage.  In addition, wooden ramps are provided at switches and crossings to ensure that low shoes do not strike rails.  Photo here.

Ramping - The noise made by the 3-phase traction control on the 1996 Tube Stock during acceleration and braking.  This is due to the GTO thyristors used on this type of control.  It does not occur on the 1995 Tube Stock because it has IGBT control.

Refuge - A temporary place of safety within a limited clearance area.

Regulating Room - London Underground speak for signalling control centre.

Regulator - London Underground speak for control room operator.

Relief - The act of changing a crew or staff at station or signal control position.  Also used as a noun as in "My relief is here".

Repeating signal - A signal that indicates the aspect of the next stop signal.  It will show a yellow caution aspect if the next stop signal is showing a red aspect or green aspect if the next stop signal is also showing a green aspect.  This is not a stop signal as it is not provided with an associated trainstop.

Reverser (1) - The traction control system device provided on a train to allow it to be driven backwards.   Usually only used in depots or under very rare emergency conditions

Reverser (2) - The name given to a train trip where the train turns back short of the end of the line as in "That train is a Colindale reverser".

Reverser Key -  Used to operate the master controller in the driver's cab where, traditionally, it was provided for Forward, Off and Reverse positions.  Also known as the "Selector Key", where it performed other tasks in addition to obtaining forward and reverse movement, on C Stock and 1967/72 Mk I & II Stocks. 

Reversing Siding - A siding, usually located between the two running lines, where trains can be turned back or reversed.  There are a number of such locations around the LU system, those in tunnels rarely used because of the slow speed enforced on the run in to a dead end now that LU can't trust their drivers not to end up in the tunnel wall.

RKL 220 key - A Yale type key which was first introduced on the 1973 Tube Stock.  It is used to open and close the Master Control Switch and which, when removed, will secure the train.  Tubeprune remembers that the first batch of these keys caused a lot of trouble because they used to break off in the lock.

Rolling Stock - has a whole page to itself here

Rounder, A - (Slang) a return trip for a train driver's single or return trip.  Also known as "A Pipe" as in "up the pipe" where a crew has to do a trip through a tube tunnel.

Running line - Any part of the railway line not in a depot or siding.


SCAT - Speed Control After Tripping - A device fitted to non-ATO trains on London Underground to ensure that a slow speed is enforced on trains which have passed a signal at danger.   The speed is limited to 10-15 mi/h for three minutes.

Scotch Block - A device used to secure a train stabled on a gradient.  It consists of a wooden block shaped to fit on the rail under the leading wheels of the train.  A handle is provided at the side to make fitting and removing the device easier.

Section Ahead/Section in Rear plungers - Equipment provided at certain tunnel stations where a substation gap is just ahead or to the rear of the station platform.  They can be used to switch off traction current.

Sectionalisation Gap - Special current rail gap at certain substations which is longer than the span of the current collector shoes of a car so that the car does not become a "bridge" between one traction current section and another.  They are normally 15 metres long.  More detail here.

Selector Key - The name used for the reverser key as used on stocks built from 1967, where it is turned upside down and inserted into a key barrel to switch in the driving control selector in the cab.  The selector key is part of the set of two (with the control key) that drivers of older stocks always have to carry with them. C Stock, A Stock, 1967 and 1972 Tube Stocks need both selector (still called the reverser key in the case of the A Stock) and control keys. The 1973 and D Stocks use the RKL220 and a control key.  The 1992s, 1995s and 1996s only need the RKL 220 on its own.  When they first appeared the RKL 220 keys used to break off in the lock and they had to be replaced by a stronger version.  They are also used for resetting passenger alarms.  See also Reverser Key.

Semi-automatic signal - A stop signal controlled from a signal box, control room or interlocking.  The signal will show a green aspect only when operated by the signaller, programme machine or computer, provided the section ahead is clear and any points have been correctly set.  The signal will automatically revert to danger when a train passes.

Senior Signal Operator - A qualified person on duty in a regulating room.  Usually known as a "regulator".

Set Number - The unique number allocated to each train in the working timetable (WTT) which identifies the trip worked by that train.  At the end of the trip the train will carry the same number through to the next trip.  The train retains the number throughout its working duty.  The number is displayed at the front and rear of the train.

Shed Receptacle - Most stabling and maintenance sheds on London Underground are not provided with current rails, so they are equipped with a shore supply system known as "overhead leads", consisting of plugs suspended from trolleys running on roof mounted rails.  The leads plug into "shed receptacles" (sometimes referred to as "receptacle boxes") provided on motor cars.  More information here

Shoe - Cast iron collector attached to insulated beams mounted on the bogies of electric trains.   The "shoe" slides on top of the rail as the train progresses so that power can be collected almost constantly.

Short Circuiting Device (SCD) - A portable device that is placed across the traction current rails to short circuit traction current.  It will switch off traction current or reduce traction current voltage in open sections and it will prevent traction current from being switched back on.  It is not used to switch off traction current except in an extreme emergency, when no other means is available.   At some locations a short circuiting device will only reduce the traction voltage, not switch it off.

Shunt Signal - A special type of stop signal which, when cleared, permits movement at caution speed into a siding or over a crossover.  Normally, trains carrying passengers are not permitted to proceed under the control of shunt signals because full interlocking may not be provided.

Siding(s) - A place where trains can normally be stabled, other than in a depot.

Signal box - A room other than the Signalling Control Centre, Stratford Market Depot Control Tower, Service Control Centre, Line Control Office, or regulating room where operating staff control the fixed signalling.

Signal Post Telephone (SPT) - A telephone located at a signal and connected directly to the signal or control centre, a signal box or a station.

Signaller - The new term for a signalman - another crude attempt at political correctness. "Signaller" is actually a rank in the Royal Navy.

Signalling or regulating room - A place where operating staff control and supervise fixed signals in areas equipped with programme machines or computer equipment.

‘Six foot’ - The space between one track and another where the lines are the normal distance apart.  

Solebar - Principal side structural member of car body shell or bogie.  On a car body, the solebar usually forms the side of the underfloor framing.

Speed inductor - A short section of dummy positive current rail that measures the speed of a train by detecting the motion of the train's collector shoes over the rail. It normally works in conjunction with a speed controlled signal.

Standard Stock - A name given to the large group of tube rolling stock built in batches between 1922 and 1934 for the Northern, Piccadilly and Bakerloo Lines.  The stock was finally withdrawn from passenger service on London Underground in 1966. The name was also used for early District stock when the stock built in 1905 for the original electrification was known as "Standard Wooden Stock" from 1910 when steel-bodied stock began to be used.

Starting Signal - The signal located at the departure end of the platform which, when cleared, permits the train to leave the station.  

Station limits - The platform area and a train's length either side of the platform.

Stick - signal (slang).  If you "hit a stick", it means you have been tripped at a red signal because you tried to drive your train past it and your train has stopped.  Nowadays it is called a "SPAD" (Signal Passed At Danger) in the UK.

Stops - Shortened name for "buffer stops".  Only referred to as "buffers" by anoraks. 

Substation - A building housing equipment for controlling the supply of traction current to sections of the railway.  More detail here.

Substation Gap (also known as "current rail gap") - location where there is a gap in the current rails between two sections fed by the substation.  Current is supplied to the current rails from substations, in sections usually about 2 - 5km long.  Some information in reply from "JDikseun" to a query on, 31 March 2002.

Suicide Pit - Explanation and Photo.  


'T' board. Shows that the headwall telephone and/or tunnel telephone wires in a traction current section might be out of order.  Should not be confused with the T sign used on the Underground to show the "termination" of a speed restriction.

Tango Coat - Orange HiVi coat worn by Train Technicians, operating officials and Instructor Operators.

Target - A portable pre-worded warning board usually used to denote some person working on or repair activity taking place on a train.  Also used to warn of scotch blocks or rail anchors applied to trains.

TCB - Ticket Collector's Box.

‘Ten foot’ - The space between one line and another where a wide space is provided between one pair of lines where there are three lines or more.

Tip out - Detrain passengers (slang).

Train Position Detector - Normally a delta track circuit, which operates when the train passes over the its location.  Used normally to trigger speed control sequences, or to release locking on backlocked signals.  A recent type of train position detector to appear is the axle counter type, where the passing wheel flanges register through a magnetic field generated by the detector. Some information in a reply from "JDikseun" to a query on, 31 March 2002.

Train Operator - The person who drives and controls the operation of the train, including those working lines where ATO (Automatic Train Operation) is provided.  Nowadays, London Underground has a requirement for about 3000 "drivers", of which, currently, around 100 are females.  There is often a shortage of train crew, amongst other staff and LU have occasional recruitment drives.  The annual salary for a train operator is currently about 30,000.  Training (described here) lasts for three months.

Track/shed switch - A three position switch on 1995/6 Tube Stock trains with three positions as follows:

  • SHED, allows 630V train equipment to be powered from the overhead trolley leads
  • OFF, isolates all 630V train equipment on the train
  • TRACK, allows 630V train equipment to be powered via the shoes.

Traction current isolating switches - Housed in yellow boxes and consisting of two switches, one positive and one negative, used to isolate a siding or terminal track from the traction current supply. These are normally always live.

Traffic Circular - A weekly internal publication listing new engineering works, signalling alterations, speed restrictions and other operational data for railway operating staff.

Train anchor (usually referred to as a rail anchor) - A device used to anchor a train or vehicle to the running rail to prevent it rolling away.  Normally used only to stable a train on a gradient.

Train stopping mark - An enamel plate consisting of a black background with a large white diamond - often mounted on a raised wooden board, in the four-foot to indicate to driver the point at which the cab should be when the train has stopped in the correct location in a platform.  Sometimes a number replaces the diamond to indicate where trains made up of differing numbers of cars should stop, e.g. 6 or 8.

Trainstop - A mechanical device fitted to all stop signals on London Underground and on Railtrack routes over which LU trains operate.  The device consists of a moveable arm which is raised when the signal is showing a stop aspect and lowered when a proceed aspect is shown.   If a train passes the signal when the arm is raised, a tripcock on the train is activated to cause an emergency application of the brakes.  This is, in effect, a mechanical form of ATP.

The design of the trainstop is fail-safe inasmuch as it it lowered, if the signal shows a proceed aspect, by air pressure against spring pressure.  A failure of air pressure causes the trainstop to rise and it is connected to the signal so that if it does rise under such a condition, the signal will show a red aspect as well as the green.  This is known as a dual aspect.

Tripcock - A mechanical device fitted to LU trains on non-ATP fitted lines, which is activated if a train passes a signal showing a danger aspect.  If activated, it causes an emergency brake application on the train.

Tripcock Tester - A gauge mounted at the trackside to allow the tripcocks of passing trains to be gauged for correct position and height.  They are spaced at intervals along the line.  On the Piccadilly Line, for example, they are located at Wood Green, Piccadilly Circus and Acton Town.  As the train enters the station, a blue lamp is illuminated near the starting signal.  If the tripcock on the train successfully passes through the tripcock tester, the lamp will be extinguished.  If the lamp remains lit, the train will be taken out of service.

Tripped - If a train passes a red signal, the emergency brake is automatically applied.  This is known as "getting tripped".  See also Tripcock and Trainstop.

Trip Valve - An electro-pneumatic valve provided on the 1967 Tube Stock which provides an emergency brake application in the event of a fault or unauthorised action detected by the train's ATP system.  It acts in the same way as the tripcock on a manually driven train.  If a 1967 Tube Stock train has to be driven over non ATO lines, tripcocks have to be fitted to the ends of the train so that it complies with the signalling system.  The trip valve is isolated under these conditions since there are no ATP codes to operate it.  A Trip Valve/Tripcock Changeover Cock is provided on the 1967 Tube Stock to allow the changeover to take place when the train is transferred from the Victoria Line to the Piccadilly Line at Finsbury Park, where crossovers between the two lines are provided.

Tunnel Headwall - The front end-wall of a platform in an underground station. Photo here.

Tunnel telephone (fixed) - A dedicated telephone, provided instead of tunnel telephone wires, fitted on the left hand side of the track at 61 metre intervals alternately placed either at cab level or at about 1.2 metres from track level and connected directly to the Line Controller.  It is used to switch off traction current in an emergency.

Tunnel telephone (train borne) - An independent communication device for use between a stationary train or vehicle and a Line Controller when it is necessary for a Train Operator to switch off the traction current supply in a tunnel section. Using the tunnel telephone for this purpose automatically connects the Train Operator to the Line Controller.  It can be used in an emergency when the train radio is inoperative.  Using the tunnel telephone for this purpose automatically switches off the traction current supply in the tunnel section.

Tunnel telephone system - A safety system used to switch off traction current in an emergency and to communicate with the Line Controller.

Tunnel telephone wires - Two parallel copper/bronze wires normally fixed along the left hand tunnel wall.  When pinched together, traction current is switched off on that section.


Ve - A standard railway term for the equilibrium speed around a curve, where the cant exactly offsets the centrifugal force.  Information in reply from "JDikseun" to a query on, 31 March 2002.

Vickers - The nickname for the Metropolitan Railway electric rolling stock built between 1927 and 1931  which, when formed into multiple unit trains with various batches of converted steam hauled coaches, became known as the T Stock in London Transport days.  Metropolitan Vickers was the name of the company which suppleid the elctrical equipment for most of these trains.

Vmax - The maximum speed that a train can safely travel around a curve (which will be higher than the Ve). Information in reply from "JDikseun" to a query on, 31 March 2002.


Wedgelock Coupler - The standard automatic coupler used on London Underground rolling stock (drawing and description here).  It was originally designed in the early 1930s, using an idea from the US based on the old Tomlinson Coupler.  The purpose of the coupler is to allow two units to be pushed together and couple automatically complete with all mechanical, electrical and pneumatic connections.  The original design allowed the electrical and mechanical connections to be controlled from a push button in the cab.  Uncoupling was also started by a push button and the two units would disconnect and push apart.  All the driver had to do was release the brakes on the train before he operated the uncouple button.

The trouble with this coupler was that, in its original form, it was too complicated.  It would only uncouple if the couplers on the two vehicles released at the same moment and this only happened if both the air operated disconnecting cylinders worked at the same time.  Often, this did not happen.   Some bizarre rituals had to be gone through to get units to uncouple, including attempts to manually lift both uncouple valves (one on each unit) with two staff calling out "one, two, three, lift" in order to make sure they did it together.  If this failed, they would drain all the air off the train and try to force the coupler pistons back against the springs by using long crowbars.  Finally, if that failed, the fitters would reverse the air pipe connections to try to force air into the uncouple cylinders permanently which the train was driven apart.  More recent versions have had some improvements, including manual operation from each unit.  Standard railway couplers would be difficult to use on tube cars because of the clearances, which is why the Wedgelock design has survived for so long.

Westinghouse Brake - The standard air brake system used by London Underground (and many other railways around the world) for about 100 years until the arrival of the 1973 Tube Stock.  It was invented in the US by George Westinghouse and was first used in the UK on the District Railway in 1875.  The system uses compressed air as the control and operating medium.  A brake pipe runs the length of the train and is used to control and supply air to the brake equipment on each car. 

Wood Line, The - Slang name for the Metropolitan Line north of Baker Street, dating from the time when the main line was the section between Paddington and Farringdon and a single track branch was opened to St Johns Wood and was called the St Johns Wood Railway.  See also The Branch.


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