Piccadilly Line
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.
The Rayner's Lane branch is definitely the poor relation - most stations don't have train time indicators, waits of 15+ minutes are not unknown, and the service seems to get suspended far more often than the Heathrow branch. A particular problem can cause massive gaps westbound in the morning between 9:30 and 10:30 when the branch service frequency drops from every 5 minutes to "10" minutes around the same time as the first shift of drivers go off duty. What is supposed to happen is that this shift end happens while the 5 minute service is still running, and alternate westbound trains for the branch actually divert to the depot at Northfields Station, hence smoothly creating a 10 minute service. What actually happens is firstly that there is a shortage of drivers, so the 5 minute service ends early, resulting in this shift end happening during the 10 minute service (hence 20 minute gaps) and secondly that some drivers are determined to go off shift with their colleagues, so that 3 "Rayner's Lane" trains in a row divert (so 20 minute gap if the 5 minute service is still running, 40 minute gap if it is not).