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
Romford–Upminster line Romford–Upminster line The Romford–Upminster line is a railway branch line in Greater London that connects , on the Great Eastern Main Line, to , on the London, Tilbury and Southend Line and London Underground. The route is in length and there is one intermediate station at . The line is part of Network Rail Strategic Route 7, SRS 07.09, and is classified as a rural line. The line is single-track throughout, electrified at 25 kV AC, has a loading gauge of W6, and a maximum speed of . As of May 2015 services are operated by London Overground, with two