Historical version 7 of Piccadilly Line (view current version)
The Tourist Tube.
The Piccadilly Line runs from Cockfosters Station in the north of London, through the centre of the the city (via Holborn Station, Leicester Square Station and Piccadilly Circus Station, down through Earl's Court Station) and the West before splitting in two; one branch goes to Rayners Lane (with some trains continuing to [Uxbridge Station]?) and the other down to [Heathrow station]? for the airport. Roughly twice as many trains go to Heathrow as to Rayners Lane.
This node is titled the Tourist Tube for fairly obvious reasons; people often end up here if they arrive by plane or by train from the North (at King's Cross St. Pancras Station), and use the line to get to hotels (there's a fairly major cluster of hotels and hostels near Russell Square Station) as well as the tourist haunts in the centre that the line serves.
There used to be a spur to Aldwych Station, accessed from a now-closed platform that seems to get a disproportionate amount of interest.