The term SUBWAY, IN LONDON can be solved by the four-letter word TUBE. The London Underground, commonly 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 TUBE is the most popular way to get around London, with over 1 billion journeys made each year.