Two of these trains were modified back to seven-car trains and transferred for service on the Circle, District and Hammersmith & City Lines in 2018. The S8 Stock entered service between 2010 and 2012, operating all services by September 2012. S Stock trains have faster acceleration than previous models, at ; their top speed of is 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 had 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 trains seat 306 passengers compared with 448 for A Stock, a reduction of 32%, but can accommodate 25% more standing passengers (1,226 compared with 976) and have dedicated space for wheelchairs. , the fleet is significantly more reliable than older trains, averaging around between failures — around seven times further than A Stock and C Stock trains, and three times further than D Stock. The voltage has been increased from nominal 630 volts to 750 volts. This allows 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. Currently, the S Stock is manually operated on most of the Metropolitan line and about half of the District line; the Circle and Hammersmith & City lines are fully automatic. The section between Latimer Road and Hammersmith was the first to be switched over to automatic operation on 10 March 2019, with further extensions being implemented in stages called Signal Migration Areas (SMA); as of January 2023 the first six SMA areas have been implemented. Due to various reasons such as funding and technical difficulties, District line automation where it shares tracks with Network Rail or Piccadilly line services has been scaled back until further notice. Features The S Stock is air-conditioned throughout: the sub-surface tunnels (unlike deep-level tube lines) allow the exhausted hot air to disperse, and two-thirds of the sub-surface network is in the open air. The stock has regenerative brakes, returning around 20% of their energy to the network and thus increasing energy efficiency. End external displays show two lines of text: the top line for the destination, and the bottom for the line. Internally, it has larger dot matrix indicators (DMIs) than D