The Great Northern, Piccadilly & Brompton Railway, today's Piccadilly Line, began life as a service between Hammersmith and Finsbury Park. It opened in December 1906. It grew to not far short of its present size in the 1930s and is an important commuter link for London's western and northern suburbs, some of which its expansion generated. Its stations include the highest number of 'listed' buildings on the Underground, with many classic Charles Holden designed examples west from Acton Town and north from Turnpike Lane. It is the only Underground line to serve Heathrow Airport and an extension to Terminal 5 was at an advanced stage of construction during its centenary year.