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
TfL Rail and in Essex. That branch comprises the first 14 stations on the Great Eastern Main Line, with interchange at Shenfield for medium- and long-distance services beyond to East Anglia. In May 2018 TfL Rail also took over operation of the Heathrow Connect service between in central London and Heathrow Airport to the west (Heathrow Terminal 4 station being the terminus). That route has nine stations. The service is operated by MTR Corporation under contract to Transport for London (TfL). Between May 2016 and May 2017, TfL Rail carried over 47 million passengers on the Shenfield branch. In June 2013 TfL