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
East Rail line branches in the north at Sheung Shui to terminate at Lo Wu or Lok Ma Chau stations. Both are boundary crossing points into Shenzhen. All of the stations are at- grade or elevated before the opening of Shatin to Central Link which will provide underground track. The distance between Hung Hom and Lo Wu stations is . The total distance of the line (including the Lok Ma Chau Spur line) is approximately , making it the longest line in total distance within the system network. The line is coloured light blue on the MTR map. The same railway is used