Circle line (London Underground) with the other London Underground sub-surface lines: the Hammersmith & City line from Hammersmith to just before Aldgate; the Metropolitan line from Baker Street to Aldgate; and the District line from Tower Hill station to Edgware Road station. The line is electrified with a four-rail DC system: a central conductor rail is energised at and a rail outside the running rail at , giving a potential difference of . The running rails are not electrified. Much of the double track railway from the Hammersmith terminus to Westbourne Park station is on a high brick viaduct. East of Westbourne Park the
London Underground D78 Stock designated as the Olympic 2012 Train with London Underground Chief Operating Officer Howard Collins carrying the Olympic Torch from Wimbledon to Wimbledon Park. This is the only London Underground train to be an Olympic Torch train. De-icing equipment was fitted to trailers 17000 to 17048 (evens). The mid-life refurbishment was the first to be carried out under the Public–private partnership (PPP), by Metronet, and was delayed until contract negotiations were completed. A prototype unit of three cars (7008/17008/8008) was prepared by London Underground's Train Modification Unit at Acton Depot in 2001. This had some detail differences from the eventual refurbishment,