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
North London line and London King's Cross stations, so since then this link has only been used for freight trains. It too was singled concurrent with the AC electrification of the eastern part of the North London line. Most of the line runs in a curve across north London. Only and stations at the western end are south of the River Thames. The river crossing is made by Kew Railway Bridge on tracks which are shared with the London Underground District line. The location of the eastern extremity has varied over the years. Between 1944 and 1986, it was at Broad Street station,