The term Londoner's Subway is a reference to the London Underground, also known as the TUBE. It is a rapid transit system serving Greater London and some parts of the adjacent counties of Buckinghamshire, Essex and Hertfordshire in the United Kingdom. The TUBE is the oldest underground railway network in the world, with services operating since 1863. It is the most extensive metro system in the United Kingdom, with 270 stations and over 400 kilometres of track.