High Quality Content by WIKIPEDIA articles! 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.