S stock

This page was last modified on 1 March 2023

The S stock forms the fleet for the Metropolitan, Circle and Hammersmith & City, and District Lines. Its rollout onto the District Line was sufficiently completed by April 2017 that the final train that it replaces could be withdrawn from surface. The S stock is now a unified stock for the entire Subsurface network.

[PHOTO: Train in station: 78kB]

Above: At eight minutes to eleven on Saturday 31 July 2010, the S stock entered service for the very first time, for semi-fast Watford trips intended for staff and their families only; many enthusiasts got wind of it, of course! The location is Wembley Park platform 1, my vantage-point is the flat roof of the former signal-cabin.

This photo was also published on page 517 of Underground News No. 585 (September 2010).


The S stock entered passenger service one train at a time from summer 2010, and by summer 2012 it had replaced the Metropolitan Line’s 1960s-vintage A stock; its replacement of the C stock was completed by summer 2014, and in April 2017 its introduction onto the District Line had spelled the end of the line for the D stock as well. Since then, the whole Subsurface network has been operated by this single type of air-conditioned train.

[PHOTO: Train in rainy station: 60kB]

Above: The author’s very first drive of an S stock arose at the conclusion of his conversion-course in autumn 2010, and comprised one trip from Neasden depot up to Watford and back. It poured with rain! In those days Train 402 was a set-number not normally used for Metropolitan Line trains, though from December 2011 any off-peak 40x train would be a Chesham-Aldgate service (or latterly Amersham/Chesham-Aldgate).


The S-stock trains are formed as a single unit, of length 8 cars (Metropolitan) or 7 cars (Circle/Hammersmith/District); these are referred to as S8 and S7 trains respectively. All axles on all cars are motored. Wide “walk-through” gangways are provided between cars, and the saloons are air-conditioned. To reduce heat/coolth wastage, saloon doors normally auto-close after 45 seconds although the driver can override this function to keep the doors open where appropriate; the override occurs automatically at Baker Street (all platforms) in a bid to reduce accidents caused by passengers rushing towards closing doors.

[PHOTO: Train interior: 64kB]

Above: A view along the inside of an S8 train on the Metropolitan Line, in which bays of transverse seating are provided in an alternating fashion. The S7 trains for the Circle, Hammersmith and District Lines have no transverse seats, allowing for greater crush-loading capacity. (21008 at Watford on 11 November 2010)

[PHOTO: Train Interior: 47kB]

Above: The gangway between cars on the S stock is wide and uncluttered, more reminiscent of a bendy-bus than of a train. (1 September 2012)

[PHOTO: Train in platform at night: 47kB]

Above: LED lamp-clusters illuminate the doorways of all active doors, a feature shared with the 2009 tube stock on the Victoria Line. (21008 at Watford on 11 November 2010)


Traction-current collection shoegear is provided on cars 1, 4, 5 and 8 in a similar pattern to the A stock.


In a departure from usual practice, compressors are carried on the driving motor cars at each end of the train (usual practice, as seen on most previous stocks, is to have the compressors on a trailer car or a non-driving car; this distributed weight more evenly along conventional trains).

Traditionally, the compressor-governor’s function is to switch the compressors ON when the Main Reservoir pressure falls below a given level, and OFF when it rises above a set maximum value.

On the S stock however, the governor’s action is incorporated into the computer; originally this was also programmed to cause the compressor to stop (or be inhibited from starting) when the train was at a station—unless the Main Reservoir pressure fell below a rather lower threshold. This was to reduce noise in platforms. However, by summer 2014 this nicety was removed from the software, as the trains turned out to be using up much more air than the designers expected—possibly because of ‘real-world’ unevenness in the track causing more work for the air-suspension—and the compressors are fairly quiet anyway!

Regenerative braking

Every axle is driven by a 3-phase AC motor which allows for rheostatic and regenerative braking; regenerative braking is already enabled, though its effectiveness depends upon other trains being nearby to “mop up” the current being fed back into the conductor-rails. Dynamic braking (whether regenerative or rheostatic) is available down to 10mph, at which point it is blended out and fully frictional braking is blended in—not very seamlessly, often causing something of a jolt which is neither the fault of, nor preventable by, the driver!

Regenerative braking is apparent, particularly to the driver but also to observers in certain locations, because a strong arc is drawn off the shoegear whilst braking. Arcing from shoegear at current-rail gaps or low-points is commonplace, but is normally observed under motoring: to see heavy arcing under braking is an unsettling sensation at first, and of course it is the rail drawing an arc off the shoegear and not the other way round!

New Signalling: Communications-Based Train Control

Until Sunday 17 March 2019, all S-stock trains in passenger service were still driven manually, with the driver responding to lineside signals.

However, a new in-cab Communications-Based Train Control (CBTC) signalling system by Thales, quite similar to that in use on the Jubilee and Northern Lines, is being introduced throughout the SSR network. Information on each train’s location and speed and its authorised speed and limit-of-movement authority are transmitted between train and a central computer via radio waves and trackside antennae.

CBTC’s rollout to trains in passenger service began on the Hammersmith & City line on Sunday 17 March 2019 from 14:00hrs, for trains in both directions in Signalling Migration Area 0.5 (SMA0.5) between Latimer Road and Hammersmith, on the Hammersmith & City line. It is possible for drivers to drive the train manually at normal speed by following the indications displayed in the cab.

CBTC’s rollout continued from 12:30hrs on Sunday 1 September 2019, when (again) in a ‘soft-launch’ passengers were allowed to board trains running on a test weekend [your editor was providing guidance to a driver at the time!], with through services commencing the next day; this introduced the new signalling to SMA1 and SMA2, which comprises the following areas:

The next area to go live, SMA3, comprised these parts of the Metropolitan / Circle / Hammersmith & City / District lines:

SMA3 went live during a final commissioning weekend on Sunday 7 March 2021.

SMA4, from Monument to Sloane Square, followed it quite soon on Saturday 24 April 2021.

SMA5, from Sloane Square to West Brompton / Barons Court / Kensington (Olympia) / Paddington (Circle) was previously expected to go live in November 2021, but actually entered revenue service at 09:00hrs on Sunday 27 March 2022. This means that the entire Circle Line is now operated under the new system.

SMA6, from Stepney Green to Becontree, went live on Saturday 14 January 2023.

SMA7, from Becontree to Upminster Depot Reception Roads, is due to go live on Saturday 18 March 2023. With District Line areas west of Barons Court de-scoped permanently, this brings to an end the District Line upgrades.

SMA8 would be on the Metropolitan Line north of Finchley Road, but if this goes ahead it will not be until 2024.


The S stock was built by Bombardier in Derby, in an order of 191 trains which will comprise 1,395 cars.

The first pre-production train was extensively tested on the Old Dalby test track in Leicestershire from February 2009, and was delivered to London in the early hours of 21 October 2009; initially it was tested on the Metropolitan Line only when segregated from passenger traffic (during engineering hours and, subsequently, during weekend shutdowns when it worked between Neasden and Watford).

On 25 June 2010 the S stock was first operated under test among passenger trains, before being made available to passengers for the first time on Saturday 31 July 2010 (see top photo). It was officially inaugurated by London’s then Mayor Boris Johnson the following Monday, 2 August 2010, as depicted below.

[PHOTO: Crowds and Mayor boarding train: 59kB]

Above: Unfortunately a glitch prevented the saloon doors being opened by the normal method when the train arrived at Wembley Park platform 1 for its inauguration by the then Mayor of London. The driver had instead to select Emergency open mode which, in those days, inhibited door-opening for the first four sets. I was well aware of this, and positioned myself thus to capture the moment at which Mr Johnson—having followed the then Chief Operating Officer Howard Collins (seen in the hi-vi) five doorways back down the platform, watched by the Press!—boarded the S stock for the very first time.

Increases in permissible and supplied traction voltages

The withdrawal of old trains and their replacement by S stock has created the opportunity to improve the efficiency of the railway; this has being done in two ways.

  1. An increase in permissible traction voltage, which saves energy by allowing more power to be fed back into the track during regenerative braking
  2. An increase in supplied traction voltage from the substations, which gives the train more power facilitating higher acceleration rates

Each of these two separate (but related) upgrades are now examined in turn.

1.  Increase in permissible traction voltage (regenerative braking ‘headroom’)

Originally the S stock could only feed electricity back into the track at 650 Volts DC. Since 2014 a programme was begun to permit this value to be increased to 790 Volts DC. This increase in permissible track-voltage allows for an corresponding increase in the useful amount of regenerative braking that can take place, and a consequent reduction in overall power-consumption.

Increases up to 790V DC of the permissible line-voltage on the SSR network took place as follows:

Further to the above, since autumn 2016, S stock trains were enabled to feed regenerative-braking current back to the track up to an further-increased upper limit of 890V DC. This means that more of trains’ kinetic energy can be harvested and passed on to other trains as traction current, thus improving electrical efficiency.

Iincreases up to 890V DC of the permissible line-voltage on the SSR network took place as follows:

Summary: S stock trains can now feed regenerative braking voltages of up to 890V DC back into the track throughout the SSR network except for Harrow to Finchley Road Met (790V DC, largely because of inter-running with the 1996 tube stock around Neasden depot), and Harrow to Uxbridge (650V DC, because of the 1973 tube stock).

2.  Increase in supplied traction voltage (substation boost)

As a separate (but related) measure, the traction current supply is also being boosted across the SSR network. The rectifiers in substations are being reconfigured so that instead of supplying the ‘LUL standard’ 630V DC, they will supply 750V DC (floated +500V and -250V). This explains why it was desirable to increase the Regen limit to 890V (see above): it was to afford sufficient ‘headroom’ for a useful amount of regenerative braking.

Increases up to 750V DC in the supplied traction voltage took place as follows:

It is estimated (by the project team, October 2017) that these increases to both supplied and permitted regen voltages could save £10million annually; my interpretation of that figure is that it would include not just savings on LU’s electricity bill, but also a reduction in maintenance arising from reduced brake-block usage and resulting decrease in tunnel dust.

Summary: The whole SSR network is now supplied by the substations at 750V DC, except for Uxbridge to Harrow to Finchley Road which remain fed at 630V DC because of the presence of 1973 and 1996 tube stocks within parts of those areas; and except for LU-fed sections west of Barons Court where inter-running of 1973 tube stock is likely to occur.

In the cab

[PHOTO: Train cab interior: 55kB]

Above: The driving controls in the cab of the S stock (unit 21004). The red handle is the fore/aft Traction Brake Controller (TBC) by which the driver controls motoring and braking. In front of the driver is the emergency stop plunger; to its left is the Signalling Interface Display (SID) which serves primarily as a speedometer under conventional signalling. The twin large monitors in front of the driver provide views of station platforms both during station-stops and until the train has fully departed the platform.

[PHOTO: Train cab interior: 53kB]

Above: At eye level to the left is a touch-screen interface with the Train Control Management System (TCMS) which controls many of the train’s systems, describes door statuses, can show CCTV images of the saloon when the train is stationary, and assists with defect diagnosis & handling. The console at top right is for the Connect Radio.

The S stock trains are equipped with an emergency detrainment device normally kept in a cupboard within the cab. It comprises two parts:

  1. Bridging Plate, which when fitted is cantilevered out from the front of the car-body; it forms either a step between two coupled trains, or the top step of the staircase (q.v.).

  2. Staircase, which slots into the Bridging Plate and is also suspended from straps which are affixed into the top of the doorway.

The detrainment device was designed by DCA, whose website hosts a video of it being deployed.