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
Transport in London the National Rail system. There are several extensions planned for the system, such as expanding its coverage and further integrating it into the Underground system. London is the focal point of the British railway network, with 18 major stations providing a combination of suburban, intercity, airport and international services; 14 of these stations are termini and 4 are through stations, while two others are both. Most areas of the city not served by the Underground or DLR are served by suburban heavy rail services into one of these stations. These suburban rail services are not part of Transport for London