London Underground S Stock

The S Stock is a class of sub-surface train currently being delivered by Bombardier Transportation in Derby to the London Underground to replace 177 existing trains on the Metropolitan, District, Hammersmith & City, and Circle lines, with a new fleet of standardised car design, totalling 191 trains or 1,395 cars. The 'S' Stock order is said to be the biggest single order of rolling stock ever made in the history of the railways in Britain.

The total cost of the S Stock trains will be £1.5 billion, according to Transport for London.

Description
The 'S' designation stands for 'Sub-surface,' following the London Underground tradition of designating surface fleets with a letter associated with its intended route. Thus, the A Stock operates the Metropolitan line to Amersham, the C Stock operates the Circle line and Hammersmith & City line, and D Stock the District line.

The stock, part of Bombardier's Movia family, is being delivered in the form of 133 "S7" seven-coach trains and 58 "S8" eight-coach trains between 2010 and 2013, and features air-conditioning throughout — the sub-surface tunnels (unlike those on the deep-level tube lines) being able to disperse the exhausted hot air, and in any case two thirds of the sub-surface network is in the open air. These trains also have regenerative brakes, returning around 20% of their energy to the network and thus reducing energy consumption.

The trains have faster acceleration than previous rolling stock, at 1.3 m/s2, but their top speed will be capped at 62 mph (100 km/h). This is 8 mph slower compared to the maximum design speed of the A-Stock (70 mph, no longer reached in normal running) but is still faster than the C and D stocks. During the period of dual operation between old and new trains, the S-stock trains will have their performance capped to match that of the older trains in order to comply with signalling constraints and avoid bunching of the service. The new trains in their 8-car configuration will accommodate 306 seated passengers compared with 448 passengers for the A-stock, a reduction of 32%; however, S-stock will be capable of accommodating 25% more standing passengers (1,226, compared with 976 in A-stock) and will have dedicated space for wheelchairs.

Eventually the line voltage on the sub-surface lines will increase from the current, nominal 630 volts to 750 volts. This will allow for better performance and also for the increased power demands of an air-conditioned, fully motored-axle train.

Detail
The S Stock will be delivered in two different types, the S8 for Metropolitan line services and the S7 for Hammersmith & City, Circle and District line services.

Both S8 and S7 Stock trains will feature an improved Customer Information System (CIS), with the front of the train showing two lines of text — the top line for the destination, and the bottom for the line. Internally, the trains have larger Dot Matrix Indicators (DMI) than the current D Stock (the A and C Stock lack any Dot Matrix Indicators at all). The DMIs show the destination and line of the train, and can display a variety of other messages, including safety ones. There are also DMIs on the exterior of the trains, with the text alternating between the destination and the line (and additionally the type of service, i.e. fast or stopping, in the case of S8 variant).

As an added feature, in order to prevent accidental pressing of the Passenger Emergency Alarms there are flaps over the alarm buttons where the wheelchair spaces are.

The air-conditioning system, the first ever on London Underground trains, is by Mitsubishi and has two circuits so that if one goes down there is still 50% aircon capacity. Through gangways from one car to the next are another innovation for LUL (though already seen on the London Overground). This provides extra room for standing passengers and creates a greater sense of security. CCTV is provided so that the driver can see into every car, while track-to-train video links enables the driver to observe the whole of the outside of the train before pulling out of a station.

The S Stock also features a fold-out set of steps in each drivers' cab to allow for a fast evacuation of the train in the event of an emergency.

S8 variant
S Stock trains of the S8 variant have a total of eight cars per train. The S8 Stock features 4 dedicated wheelchair spaces in all trains, and cantilever seating for easier cleaning and more accessible storage of bags.

The external displays on the S8s alternate between the destination, the type of service and the line name. E.g. - 'Watford', 'All stations', 'Metropolitan line'. The S Stock is the first fleet to be used on the Metropolitan line that features Dot Matrix Indicators and Digital Voice Announcements. The S Stock trains on the Metropolitan line utilise the same announcer voice as the 2009 stock on the Victoria line and the C Stock on the Circle, District, and Hammersmith & City lines.

S7 variant
The other type will be known as the S7 Stock — which will be used by the Circle, District and Hammersmith & City lines. It is much like the S8 Stock, except that trains have seven cars instead of eight, and (in keeping with its use on inner-city lines with higher short-distance ridership) will have longitudinal seating throughout, and no transverse seating.

As of October 2011, one S7 train has been in London for testing purposes. This is not the final design, as it is an S8 train with a carriage temporarily removed.

Entry into service
The S Stock was initially tested overnight between and  via the Watford North Curve from 9 November 2009. Driver training began in early January 2010, and the first train entered revenue service on the Metropolitan line on 31 July 2010 shuttling between Wembley Park and Watford.

By 27 June 2011, the S Stock was running across the whole Metropolitan line network. As of 14 October 2011, 22 trains had been delivered to Neasden Depot (two pre-production trains re-modified to S7).

Transport for London expects all 58 S8 trains for the Metropolitan line to be in service by summer 2012.

The first of the 191 S7 trains will enter service on the Circle and Hammersmith & City lines, starting later in 2012. Last to arrive will be those for the District line, starting in 2013, with the whole S stock fleet in service by 2015. This is primarily because the District line's existing (D78 Stock) trains were refurbished relatively recently, whereas the 43-year-old C stock trains on the Circle and Hammersmith services are "in an increasingly poor state" according to LUL. Also, it is hoped that the Circle and Hammersmith routes will have some new trains in service by summer 2012 in order to maximise passenger capacity on the northern half of the Circle line in time for the Olympic Games. They are longer than the trains they are replacing (seven cars and 117m long instead of six cars and 93m long), so certain station platforms are having to be lengthened before they can operate in normal service. For stations where this has proved physically impossible, such as Baker Street, the trains have a selective door-opening capability whereby the set of doors at the end of the train will not open.

By November 2011, S Stock deliveries were suspended by Transport for London due to concerns over reliability. A number of train sets had been delivered to London Underground's Neasden depot, but had not been accepted to enter service. Deliveries resumed in mid-December 2011.