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
Circle line (London Underground) jointly-owned six-carriage train began passenger service in 1900. Following this an AC system was suggested, and this was accepted by both parties. However, the District was looking for a way to raise the finance needed and in 1901 found an investor, the American Charles Yerkes. He formed the Underground Electric Railways of London (UERL), and his experience in the United States led him to favour DC, with third-rail pick-up similar to that in use on the City & South London Railway and Central London Railway. After arbitration by the Board of Trade the DC system was taken up, and the