Mind the Gap!
The London Underground is a rapid transit system serving a large part of Greater London and neighboring areas of Essex, Hertfordshire and Buckinghamshire in the UK. The first section opened in 1863, and was the first underground railway system in the world, and, starting in 1890, it was also 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, 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. Wiki - London Underground