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C Stock

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London Underground C Stock

The C Stock was introduced into passenger service in 1970 and is the principal stock used for services on the Circle and Hammersmith Lines.  C Stock is also used on the District Line services on the Wimbledon to Edgware Road route.   This page looks at the C Stock in some detail.  Information and many photos supplied by District Dave.  Further information supplied by Tube Troll.


C Stock - Train Design - Cab & Controls - Modifications & Refurbishment - Doors - Public Address - Equipment - Brake testing - Maintenance

C Stock

C Stock orig unit.jpg (52064 bytes)Fig 1:  C69 Stock as originally delivered in 1970.

Click on the image for the full size view and description.

The "C" is for "Circle".   The C Stock is used on London Underground's Hammersmith & City and Circle Lines.  These two lines have always been closely linked, ever since a uniform type of stock was introduced on the lines in the late 1930s.   The C Stock replaced this older stock, called the CO/CP Stock, from 1970-71.   The displaced cars were transferred to the District where they replaced even older Q Stock.  At this time, the C Stock was officially called C69 Stock, the "69" indicating the year of its ordering.  Originally, it only worked on the Hammersmith and Circle Lines.  Its use on the District was not to come for several years more.

C Stock orig interior.jpg (84824 bytes)Fig 2: C Stock interior as originally delivered

Click on the image for the full size view and description.

The trains were built by Metro-Cammell (now Alstom) in Birmingham and were towed by rail to the London Underground depot at West Ruislip.  They were commissioned there before transferring to Hammersmith Depot to enter service. 

A second batch of C Stock, known as the C77 Stock, was ordered for the District Line's Edgware Road to Wimbledon service in 1977.  These trains were ordered to allow replacement of all the old cars operating on the District - the R Stock and the CO/CP Stocks.  The plan at that time was to operate the District main line services with new D Stock - which was designed with longer than standard bodies - and the Edgware Road route with short-bodied cars - the C Stock.  The reason for this mix was that the Edgware Road route has short platforms at Notting Hill Gate, Bayswater and Paddington and longer cars could not be accommodated at these stations and hence the increase in the C Stock requirements.

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Train Design

The C Stock is formed of 2-car units comprising a motor car and a trailer car.  The motor car has a cab but there is no cab on the trailer.  The two cars are coupled by a semi-permanent bar coupler.   At the outer ends of the unit, a full automatic coupler is provided.  A standard 6-car train is formed by coupling three of these 2-car units.  The tare weight of the train is 155.7 tons.

The cars were originally referred to as DM and UT.  The DM was "Driving Motor" car, while the UT was "Uncoupling Trailer".  The U was soon dropped and trailers are normally referred to as just T.  The outer end of the trailer originally had no facilities for driving.  If it was necessary to move a unit with the trailer leading, a shunter had to stand at the end door and shout instructions to the driver in the cab at the other end of the two cars.  There were no personal radios in those days.  The driver drove the train in reverse, looking back down the unit.  This may seem a bit primitive these days but it was not considered too difficult once you got used to it and accidents were few and far between.  This arrangement lasted until the stock was refurbished in 1990-94, when a shunting controls were fitted to the outer ends of trailers.

The C69 Stock originally comprised 106 motor cars and 106 trailers, numbered 5501-5606 and 6501-6606 respectively.  Units were formed M-T with matching numbers, e.g 5501-6501.  Trains were formed as M-T + M-T + T-M or M-T + T-M + T-M.  The middle unit can face either way round.  Fourteen Trailers (6543-6556) were delivered with de-icing equipment but three of these have since been de-commissioned (cars 6554-6556).  The C77 Stock was numbered 5701-5733 and 6701-6733 (total 66 cars) and was almost the same as the C69 cars except that they were delivered with white roofs instead of the usual black and a few small interior modifications.  Since refurbishment in the early 1990s, they are indistinguishable from each other.

One additional car was added to the C77 order to replace C69 car 5585, which was damaged in the West Ham terrorist bomb incident in March 1976.   The driver was killed.   Unit 5585-6585 thus became a hybrid C69/C77 unit and was selected for a trial refurbishment in 1989, which was carried out at BREL, Derby.

One unit of C Stock has been scrapped.   This was 5606-6606, which was fitted experimentally in 1974 with traction equipment supplied by Kiepe.  Eventually, spares became difficult to obtain and maintenance was a constant source of problems.  The unit was withdrawn in 1991 and was scrapped in 1993.  There are now 276 vehicles making up 138 units or 46 x 6-car trains.

Couplers are "universal", i.e. they are designed to allow units to be coupled either way round.  This was a new idea for the Circle Line at the time because the old trains were "handed" and could only couple "right way round.  Services had to be scheduled so that all trains arrived at Hammersmith Depot facing the same way.  See Coupling, Handing and UNDMs for the story of how this all worked. 

One of the problems of working the Circle Line was that wheel wear was always one-sided.  Trains always ran round the Circle facing the same way.  It was necessary to lift cars every few months to allow the wheelsets to be swapped round to even out the wear.  When the C Stock was introduced, the universal automatic couplers allowed trains to be scheduled to turn.  This was achieved by running two Circle trains each day from Tower Hill to Whitechapel, where they reversed and then went to Hammersmith to stable after the evening peak.  This stopped the uneven wheel wear and gave considerable savings in maintenance work.

Car Weight Seats Equipment
DM 31.7 tons 32 Cab Shoes Traction Package Automatic coupler Motor Alternator   Batteries
T 20.2 tons 32 Shunting Controls     Automatic coupler   Compressor  

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Cab and Controls

C Stock cab orig.jpg (66649 bytes)Fig 3: C Stock cab in original condition

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The C Stock cab arrangement was originally designed for Automatic Train Operation and the arrangement of the controls is similar to the 1967 Tube Stock built for the Victoria Line.  The traction/brake controller is on the left hand side of the control desk and the selector key on the right.  In the centre of the desk are the air gauges and speedo with some indicator lights.  It was intended that the ATO code indications would be displayed here too.  After a few months in service, it was found that the display had to be protected by a transparent plastic cover, which unfortunately acts as a reflector in bright sunlight, making the display almost invisible.

At the outer end of each trailer car, there is a shunting control cabinet which is used to allow the unit to be driven from the non-driving end at reduced speed in the depot.  The cabinet contains a set of miniaturised controls for selection of direction speed and braking.  There is also a whistle and an air gauge.  The cabinet is normally kept locked behind a panel on the left side of the car end.  This equipment was only fitted during the refurbishment programme of the early 1990s.  Before this time, all shunting was carried out from the driver's cab, even, when required, in reverse.

C Stock Cabpc.jpg (61956 bytes)Fig 4: Interior of the present C Stock cab

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There have been some modifications to the stock since it entered service.  The original set up for the trains was for two-person operation.  The guard occupied the rear cab and controlled the doors from there.  The trains were converted to OPO in 1985.  The conversion consisted of minor switching modifications to the trains but required the installation of train radio and mirrors and CCTV cameras on stations. 

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Modifications & Refurbishment

The C Stock has undergone a number of "mods" during its lifetime.  Obviously, the refurbishment programme produced the largest number of changes but there were others carried out over the years before and since, for example, new cab seats were fitted in 1986.  The cab controls were modified from time to time and a number of technical modifications were introduced over the years, particularly as a result of the OPO conversion of 1984.  At that time, the original door controls on the rear walls were used by drivers at each station.  For platforms on the offside, drivers had to cross the cab.  For nearside platforms, some drivers used to operate the controls seated, meaning that they had to push the buttons immediately behind them with their arm behind their back.  Of course, the door controls were never meant to be used in this way - they had been designed with ATO in mind.  As the trains were still being driven manually, time was lost at stations as drivers had to leave their driving position to operated the doors.  In 1986-7, door controls were added to the driver's control desks, see Fig 6 below.

C Stock leaving Barbican.JPG (58855 bytes)Fig 5: Refurbished C Stock at Barbican

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The refurbishment was carried out in a response to the Kings Cross fire of November 1987.  It was decided that materials used inside cars could be further fireproofed and that the opportunity to improve the ambience should be taken at the same time.  A pair of cars was sent to BREL at Derby in 1989 for an experimental refurbishment.  Each car was done differently but neither was considered a sufficient improvement over the existing interior.  Eventually, a new scheme by a design house known as CreActive was selected for the fleet refurbishment, which was carried out by RFS Engineering (Doncaster) Ltd. between 1990 and 1994.

During refurbishment, the glass fibre roof domes at the ends of cars were replaced by phenolic ones.  This was in the belief that the fire resistance of the old roof domes was insufficient.  There was a lot of fire protection hysteria in London Underground after the 1987 Kings Cross fire and this was a good example.  Also during refurbishment, new ventilation slots were cut into car roofs.   These were put in place for an improved forced air ventilation system was planned for the trains to replace the original system which was unreliable.  It has yet to be fitted.

C Stock Interior daylight DD.jpg (41828 bytes)Fig 6: C Stock refurbished interior.

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The biggest change was in the seating layout, where the transverse seating was changed to longitudinal.  The new arrangement gives a wider gangway along the car and gives a more open appearance.  However, the standback spaces at the doorways were lost and this has restricted the boarding and alighting times somewhat.  The same number of seats is provided where the transverse seats existed but the seat width is slightly narrower.  Another attempt to provide a more secure ambience inside cars is the provision of end windows.  Remote trip reset and speed control after tripping (SCAT) were also been added to the stock as part of the refurbishment programme.

C Stock bogie orig.jpg (85699 bytes)Fig 7: C Stock bogie as originally delivered

Click on the image for the full size view and detailed description.

The bogies were also extensively modified during refurbishment and the original air "Metacone" suspension was removed and replaced with plain rubber suspension.  The bogies were also provided with new frames.

The C Stock is now limited to 40 mph (although not governed) and most cabs have a sticker to this effect on the gauge panel.   Operating instructions for the stock state that the Rate Switch (which controls acceleration) must only be used in Rate 1 and the Weak Field Flag is never raised.   The combination of this tends to limit the speed, although 40 mph + is possible, for example between East Putney and Southfields and Southfields and Wimbledon, both westbound.  The Rate 2 acceleration level was always too high for the stock and often caused wheelslip.

C Stock at S Ken WB.JPG (34499 bytes)Fig 8: C Stock at S Kensington with added mouldings on the cab corners for inter-car barriers used when the unit is coupled in the middle of a train.

Click on the image for the full size view and description.

Inter-car barriers (ICBs) have been provided to prevent persons falling between cars at stations.   The barriers are black canvas hung on brackets fitted to the outer corners of the car body.  They are coupled between cars on sprung attachments to allow transition on curves without creating a gap or tearing.  They have become a maintenance problem and are a nuisance when coupling and uncoupling cars.

In recent years, the original half-spherical door fault indicator lights, fitted to the inner ends of cars to show operators which cars have doors open, have been replaced by electronic lamps on small rectangular housings.  

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The C Stock was the first stock to have powered driver's cab doors.  The doors operated in the same way as the passenger doors but were separately controlled from the cab.  It was decided that the doors should have interlocks to prevent the train being moved with either cab door open.  This was strange because most passengers assumed that the train would not start with the passengers doors open.  This was not the case under two-person operation.  Only the cab doors were interlocked in this way.  This remained the system until OPO was introduced from 1984.  The logic behind this regime was founded on the premise that the passenger doors did not need to be interlocked with train movement because there were two people checking all doors were closed - the guard with his "pilot light" and the driver waiting to hear the bell before starting.  It worked well enough for 80 years.

Each C Stock car has four double doorways on each side.  The doorway opening is built to the traditional London Underground 5 foot dimension.  The door arrangement is designed to allow rapid loading and unloading at stations and is one of the better features of the stock.  Door engines are the standard London Underground air operated type.  The large number of door openings required that the car structure be designed with 300 mm (1 foot) deep solebars.

C Stock control desk.jpg (27925 bytes)Fig 9: C Stock modernised cab control desk .

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The door controls include Selective Close, which allowed all doors except one pair on each car, to be closed.  It is useful in allowing heat to be retained inside the cars when standing at terminals for a long time.   Following refurbishment, door control has been fitted with "close chimes", which sound for two seconds as doors close.

C Stock offside rear dooor controls.jpg (53982 bytes)Fig 10: C Stock door controls

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Public Address

C Stock PA panel.jpg (13560 bytes)Fig 11: C Stock public address controls

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At refurbishment, the C Stock was provided with an automatic public address system.  It allows the driver to select a route and for the PA system to announce the stations automatically as the train progresses along the route.  It is known as the DVA or Digital Voice Announcer.

The controls allow the driver to select a route, select peak and off-peak announcements and even announce a "closed station".  Special routes can also be programmed separately.

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The C Stock is equipped with DC traction motors using a pair of pneumatically driven camshafts, one series and one parallel, for resistance control.  The traction circuit is arranged in a classic series-parallel configuration with weak field control.  Each motor car in a 2-car unit has four motors, giving 50% axles motored in a train.  The two motors on one bogie are permanently connected in series.   The C Stock motors are type LT 117, built by Brush and drive the axle through a gear ratio of 17:114.  The gear ratio was designed to give high acceleration but it sacrificed speed.  The stock was originally restricted to a 45 mph top speed, only exceeded on the occasional trips to Amersham when the stock was used to cover for long gaps in the Metropolitan service during the staff shortages of the early 1970s.  The pinion design to achieve this gear ratio was new and led to excessive wear, which resulted in the early renewal of the pinions.

The low voltage traction control circuits are duplicated and arranged so that one set of control circuits operates alternate motor cars along the train - i.e. two out of three - while the other set operates the third motor car.  This arrangement ensures that a low voltage control circuit failure will not disable the train.  The control circuit alternatives are selected by a Fault Isolating Switch (FIS) provided in the cab.

Each 2-car unit has a single reciprocating air compressor mounted under the trailer car. The compressor is of either the Westinghouse 3HC43 or Reavell TBC38Z type.  The Reavell compressor is quieter.  As is standard on the London Underground, compressor operation is synchronised, so that all compressors switch on and off together. 

Each motor car has a motor alternator (MA) providing 230 volts AC at 850Hz for auxiliary services.  The output is stepped down to 60 volts AC for the battery charger and to 115 volts AC for the main car lighting.   The 60 volt AC rectifier output recharges the battery on each motor car at 50 volts DC and supplies the control circuits at that voltage.  The basic system is shown in the diagram Fig 13 below.

C Stock Power Schematic.gif (19107 bytes)Fig 12:  C Stock equipment schematic, showing a simplified general arrangement of the on-board systems in use today.   Note that there have been a number of changes in equipment since the stock entered service.

Click on the image for the full size view.

The C Stock is equipped with four braking systems - a spring applied parking brake (SAPB), the Westinghouse air brake (used for safety systems and as a back-up, see the red train line pipe in Fig 13 above), the electro-pneumatic brake and a dynamic brake.  Originally the stock had a hydraulic, manually applied parking brake, of a type also used on some BR trains of the 1960 and 70s.  It was a constant source of trouble.  Leaks caused the brake to apply and the train then had to be dragged to get it moving.  Some very deep flats resulted from such excursions.  Also, the fluid used to get everything nearby under the car sticky and caused a fire hazard.  It also left a nasty slippery track when it leaked.  It was replaced with SAPBs at refurbishment.  The spring applied parking brake is automatic.  It will apply blocks to the motored axles if brake cylinder air falls below a predetermined level (1.5 bar) while both main reservoir pressure and train line pressure falls below pre-set levels.  The only snag with it is that if main reservoir air is lost on a car or unit, the cars with SAPBs applied have to be dragged with brakes applied.   The wheels will turn but you shouldn't go too fast for fear of overheating them.  

C Stock CTBC.jpg (29656 bytes)Fig 13:  C Stock combined traction/brake controller (CT/BC) showing the operating positions.

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The Westinghouse brake works under the same principles as all air pure brakes but, on the C Stock, uses diaphragm type triple valves.   The electro-pneumatic (e-p) brake is is overlaid on the Westinghouse system and is a development of the system designed for the 1967 Tube Stock ATO brake system.  There are four brake application steps, each controlled by mercury retardation controllers.  When the train is standing, a "holding brake" can be selected to keep a small brake cylinder pressure (1 bar) by means of a pressure switch system.

The e-p brake operates in conjunction with the dynamic brake.  The traction motors are used as generators to provide a rheostatic brake on the train.  The energy produced is dispersed as heat in on-board resistors.  A blending system allows the rheostatic brake to provide the required level of braking on the motor cars with the e-p brake acting on trailers as necessary.  When the train reaches about 20 mph, the rheostatic brake starts to fade and the air brake takes over to provide the full braking effort required.  Well, that's the theory.  In reality, it does work but it is a bit sloppy in control.

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Brake Testing

During its refurbishment, the C Stock was fitted with a brake test system, which is controlled from the offside console in the driver's cab (Fig 14 below).  It has never been formally introduced into use by train staff.  It seems that it was intended to allow trains to be prepared for service by one person but, presently, C Stock trains are officially prepared by two people.  A bit strange, that.

C Stock offside console.jpg (59540 bytes)Fig 14:  Offside console controls

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The C Stock is maintained at Hammersmith Depot, even those trains used on the District Line, which transfer to the depot from Edgware Road.  The stock is now London Underground's most unreliable stock, only occasionally reaching 5,000 kms between service failures.  It is usually below this.  The stock has always been troublesome.  There were a number of features, like the Metacone suspension and the load/weigh control which proved difficult to maintain.  These were removed at refurbishment but, even so, the reliability has not improved much. 

C Stock Tower Hill 2.JPG (50710 bytes)Fig 15:  Rear of C Stock train on Circle Line at Tower Hill (EB or Inner Rail).

Click on the image for the full size view.

There is also the problem of doors on this stock, which add to the reliability problems.   The C Stock has 25% more doors than any other stock and doors generally account for about half all train failures.  This means that the C Stock is more vulnerable to door failures.  The stock also has one of the hardest duty cycles on the Underground, the Circle Line.


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