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
Wimbledon and Sutton Railway any construction and so extensions to the earlier acts were granted each year from 1918 to 1922 to give a final date of 26 July 1924 for completion of the compulsory purchase. In November 1922, notices of new bills to be placed before Parliament were published by the W&SR, and by the UERL's subsidiaries the London Electric Railway (LER) and the City and South London Railway (C&SLR, now part of the London Underground's Northern line). Taken together, the bills brought significant changes to the plans for the Wimbledon to Sutton line. The C&SLR was an underground railway running in deep