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.