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 and is 44 miles long. 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 tracks with the Piccadilly line. Printed in dark blue (officially "Corporate Blue", Pantone 072) on the Tube map, it is the fourth-busiest line on the Underground network with over 210 million passenger journeys in 2011/12.

The first section, between Finsbury Park and Hammersmith, was opened in 1906 as the Great Northern, Piccadilly and Brompton Railway (GNP&BR). The station tunnels and buildings were designed by Leslie Green, featuring ox-blood terracotta facades with semi-circular windows on the first floor. When Underground Electric Railways of London (UERL) took over the line, it was renamed the Piccadilly line.

 

The line has two depots, at Cockfosters and Northfields. The former site is near Trent Park, and was the preferred site over Oakwood, which was smaller, and its only access point was south of the station. Subsequently, Oakwood was built with an island platform, with its intended three-track terminus layout moved to Cockfosters. Light maintenance and cleaning of trains is done here, with the latter site, being the main depot, having train maintenance facilities. Northfields depot is also double-ended, with access from just west of Boston Manor station and Northfields station. For it to be built, the latter station had to be rebuilt nearer to South Ealing station. The depot was opened earlier than the Hounslow extension, on 4 July 1932, and fully electrified two months later. Lillie Bridge was the main depot when the Piccadilly line was initially opened.