The London Underground is a metro system serving a large part of Greater London and neighbouring areas of Essex, Hertfordshire, Kent and Buckinghamshire in the UK. It is both the world's oldest underground railway and the oldest rapid transit system. It was also the first underground railway to operate electric trains. It is usually referred to as the Underground or the Tube - the latter deriving from the shape of the system's deep-bore tunnels although some 55% of the network is above ground.