A shared station at Heathrow Airport opened 27 March 2008, served by Heathrow Express and Piccadilly Line trains. It is in Travelcard Zone 6.
The London Underground section of the station is situated in Travelcard Zone 6; it is the westernmost below-ground station on the network. It is also the westernmost National Rail station in London.
More information on the Wikipedia page [1]. The TfL page is [2].