The S Stock is a fleet of subsurface rolling stock utilized on the London Underground since 2010. Delivered by Bombardier Transportation in Derby, the S Stock was ordered to replace the A60, A62, C69, C77 and D78 stock on the Metropolitan, District, Hammersmith & City, and Circle lines. The order consists of a standardised fleet of 191 trains (1,395 cars), with 1 additional train (8 cars) ordered as a part of Metropolitan line extension to Watford Junction (192 trains = 1,403 cars). The stock has two types, S7 and S8, serving different lines, with differences in the arrangement of seating and number of cars. Both types have air-conditioning and low floors to ease accessibility for disabled people, and it is also possible for a passenger to walk from one car to the next while the train is moving, unlike older trains on the network.
The order is said to be the biggest single rolling-stock order in Britain. According to Transport for London, the cost will be £1.5 billion.
Passenger service began on the Metropolitan line in July 2010, the Hammersmith & City line in July 2012, and the Circle and District lines in September 2013. The S Stock completely replaced the A Stock on the Metropolitan line in September 2012 and the C Stock on the Circle and Hammersmith & City lines in February 2014, and started operating on the District line in June 2014; it is expected to fully replace the D Stock on the rest of the District line by December 2016.
The S designation stands for sub-surface, following the London Underground tradition of designating surface stock with a letter associated with its intended route – A Stock on the Metropolitan line to Amersham, the C Stock on the Circle line and Hammersmith & City line, and D Stock on the District line.
Part of Bombardier’s Movia family, the stock consists of 133 S7 seven-car trains for the Circle, District and Hammersmith & City lines and 58 S8 eight-car trains for the Metropolitan line. There is also one additional eight-car train called an S7+1 in use on the Metropolitan line, ordered as part of the Metropolitan Line extension to Watford Junction. This train is formed of a seven-car S7 and an additional car from another S7, forming an eight-car train. This train, however, retains the all-longitudinal seating of the S7, and thus can be distinguished from the remaining S8’s. The S8 Stock entered service between 2010 and 2012, operating all services by September 2012.
The stock has faster acceleration than previous stock, at 1.3 m/s2 (2.9 mph per second), but its top speed is 62 mph (100 km/h), 8 mph slower than A Stock but faster than C and D Stocks. During the period of dual operation with both old and new trains, the stock has its performance capped to match that of the older trains in order to comply with signalling constraints and avoid bunching of the service. S8 stock seats 306 passengers compared with 448 for A Stock, a reduction of 32%, but it can accommodate 25% more standing passengers (1,226 compared with 976) and has dedicated space for wheelchairs.
Eventually the voltage will be increased from nominal 630 volts to 750 volts. This will allow for better performance and also for the increased power demands of air-conditioned, fully motored-axle trains, and allow the trains to return energy to the network through regenerative braking.
At present the stock is operated manually, but London Underground will activate automatic train operation once the necessary signalling is in place by 2022. It was initially intended that the upgrade would happen in 2018, but has been delayed
by simon schofield