The London Underground, often referred to as 'The TUBE', is a rapid transit system serving a large part of Greater London and parts of the adjacent counties of Buckinghamshire, Essex and Hertfordshire 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, and in 2019/20 carried 1.357 billion passengers, making it the 11th busiest metro system in the world.