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
History of the London Underground were first installed at Earl's Court between the District and Piccadilly line platforms, and at all deep level tube stations after 1912. The last, the Charing Cross, Euston & Hampstead (the Hampstead) opened in 1907, and ran from to Camden Town, before splitting into two branches going to Golders Green and Highgate (now Archway). To promote travel by the underground railways in London a joint marketing arrangement was agreed that included maps, joint publicity and through ticketing. UD signs were used outside stations in Central London. The UERL acquired London bus and tram companies in 1912 and the following year