S Stock sub-surface trains, delivered by Bombardier Transportation in Derby to the London Underground, are in the process of replacing 177 trains on the Metropolitan, District, Hammersmith & City, and Circle lines. The order consists of a standardised fleet of 191 trains (1,395 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 possible to change cars when the train is moving, unlike older subsurface trains.