The Underground Electric Railways Company of London Limited (UERL), known operationally as the Underground for much of its existence, was established in 1902. It was the holding company for the three deep-level "tube" underground railway lines opened in London during 1906 and 1907: the Baker Street and Waterloo Railway, the Charing Cross, Euston and Hampstead Railway and the Great Northern, Piccadilly and Brompton Railway. It was also the parent company from 1902 of the District Railway, which it electrified between 1903 and 1905. The UERL is a precursor of today's London Underground; its three tube lines form the central sections of today's Bakerloo, Northern and Piccadilly lines.

The Piccadilly line is a deep-level London Underground line running from the north to the west of London. It has two branches, which split at Acton Town, and serves 53 stations. The line serves Heathrow Airport, and some of its stations are near tourist attractions such as Piccadilly Circus and Buckingham Palace. The District and Metropolitan lines share some sections of track with the Piccadilly line. Printed in dark blue on the Tube map, it is the sixth-busiest line on the Underground network, with nearly 218 million passenger journeys in 2019.