This page was last modified on 14 March 2017
The S stock forms the fleet for the Metropolitan, Circle and Hammersmith & City Lines, and for District Line services between Edgware Road and Wimbledon. Its rollout onto the remaining District Line services is largely complete: it will then be a unified stock for the entire Subsurface network.
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 its introduction onto the District Line will soon spell the end of the line for the D stock as well. The whole Subsurface network will then be operated by this single type of air-conditioned train.
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.
Traction-current collection shoegear is provided on cars 1, 4, 5 and 8 in a similar pattern to the A stock.
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!
Since February 2014, the Metropolitan Line’s S stock can feed current back into the track at up to 790 Volts DC (increased from 650 Volts), between Finchley Road and Amersham/Chesham/Watford; from summer 2014 (with the removal of the C stock) this area has been extended to Moorgate, Hammersmith and Notting Hill Gate. 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.
Further increases (to 790V DC) to the permissible line-voltage on the SSR network took place as follows:
Additionally, as of October 2016, S-stock trains on the Metropolitan Line north of Harrow-on-the-Hill 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.
Further increases (to 890V) to the permissible line-voltage on the SSR network took place as follows:
As a separate (but related) measure, the traction current supplied by the rectifiers in Metropolitan Line substations north of Harrow-on-the-Hill to Amersham was boosted (on 15 October 2016) from the usual 630V DC up to 750V DC (floated +500V and -250V); this explains the desire to increase the Regen limit to 890V, to afford sufficient ‘headroom’ for a useful amount of regenerative braking.
Further increases (to 750V DC) in the supplied traction voltage took place as follows:
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 evens out imbalances in weight distribution along the train).
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!
The S stock is being 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 Mayor Boris Johnson the following Monday, 2 August 2010, as depicted below.
The S stock trains are equipped with an emergency detrainment device normally kept in a cupboard within the cab. It comprises two parts:
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.).
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.