The Piccadilly line is a line of the London Underground. It is coloured dark blue on the Tube map. It is the fifth busiest line on the Underground network judged by the number of passengers transported per year. It is mainly a deep-level line, running from the north to the west of London via Zone 1. However, there are a number of surface sections mostly in its westernmost parts. Out of the 53 stations served, 25 are underground. It is the second longest line on the system, after the Central Line.