The term London Underground is commonly referred to as the TUBE, which is a four letter word. The London Underground is a rapid transit system serving a large part of Greater London and some parts of Buckinghamshire, Hertfordshire and Essex in the United Kingdom. It is the oldest underground railway network in the world, with the first section opening in 1863. The network has expanded to 11 lines, 270 stations and 402 kilometers of track. It is one of the most used metro systems in the world, with over 1.3 billion journeys made in 2018/19.