Despite being named under the same system as London Underground lines, and having sections which are underground, the Elizabeth line is not classified as a London Underground line. Under the project name of Crossrail, the system was approved in 2007, and construction began in 2009.

No, the Elizabeth line in London is not entirely underground; in fact, only a relatively small portion of its total 60-mile (100 km) length is located in tunnels. The line is divided into three sections: the western section from Reading and Heathrow, the central section through the heart of London, and the eastern section to Shenfield and Abbey Wood. The central section is the part that is entirely underground, featuring 26 miles (42 km) of newly constructed twin-bore tunnels that run between Paddington in the west and Abbey Wood and Stratford in the east. This core section includes the deep-level, cathedral-like stations such as Canary Wharf, Liverpool Street, and Tottenham Court Road. However, once the trains move outside of this central core, they transition to surface-level tracks that were largely pre-existing as part of the Great Western Main Line and the Great Eastern Main Line. Approximately 75% of the line’s route is above ground, where the trains share tracks and infrastructure with other National Rail services. This hybrid design allows the Elizabeth line to function as a high-frequency "metro" in the city center while acting as a fast "commuter rail" in the outer suburbs, providing a seamless journey across the South East of England.