The London Subway System, also known as the UNDERGROUND, is a rapid transit system serving a large part of Greater London and parts of the adjacent counties of Buckinghamshire, Essex and Hertfordshire. It is the oldest UNDERGROUND railway network in the world, with the first section opening in 1863. The network has expanded to 11 lines, with 270 stations and over 400 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.