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
Urban rail in the United Kingdom Line, via tunnels through central London, with a branch to . It is the largest construction project in Europe. Crossrail will start a full service in 2018, taking over suburban services operated by Abellio Greater Anglia to Shenfield, Heathrow Connect to Heathrow Airport and First Great Western to Reading. Once opened, Crossrail will be one of three partially underground suburban rail networks in the United Kingdom as the network cannot be classed as a Metro system like the London Underground or the Glasgow Subway. The Merseyrail and Glasgow commuter rail networks also have below ground stations. Crossrail 2 is a