Heathrow Terminal 4 is a Piccadilly line Zone 6 station serving the terminal used primarily by Sky Team alliance airlines including Air France, KLM and Delta. The station is on a separate loop from the main Heathrow Terminals 2 & 3 and Terminal 5 stations, requiring careful planning to use. Not all Piccadilly line trains stop here -trains alternate between the T2/3 loop and the T4 branch.