The London Underground is a rapid transit system serving a large part of Greater London and neighbouring areas of Essex, Hertfordshire and Buckinghamshire in the UK. With its first section opening in 1863, it was the first underground railway system in the world. In 1890 it became the first to operate electric trains. Despite the name, about 55% of the network is above ground. It is usually referred to officially as 'the Underground' and colloquially as the Tube, although the latter term originally applied only to the deep-level bored lines, along which run slightly lower, narrower trains along standard-gauge track, to distinguish them from the sub-surface "cut and cover" lines that were built first. More recently this distinction has been lost and the whole system is now referred to as 'the Tube', even in recent years by its operator in official publicity.
The earlier lines of the present London Underground network were built by various private companies. Apart from the main line railways, they became part of an integrated transport system in 1933 when the London Passenger Transport Board (LPTB) or London Transport was created.
