From the Wikipedia page 
Heathrow Central is a railway station located in London Heathrow Airport. It was formerly known as Heathrow Terminals 1, 2, 3 and the station signage has the subtitle Terminals 1, 2 & 3.
The station is served by Heathrow Express, normally running every 15 minutes and Heathrow Connect services, running half-hourly. The former run non-stop to Paddington, whilst the latter stop at most intermediate stations. Heathrow Express also run a shuttle service from Heathrow Central to Terminal 4 to provide four trains an hour between these stations (a handful of weekday and most Sunday services are operated by Heathrow Connect).
Transport for London Travelcards are not valid on services from the station. Passengers transferring between any of the terminals at Heathrow may use the trains free of charge.
With the opening of Terminal 5 on 27 March 2008, Heathrow Express services now only run to Heathrow Terminal 5, with the Heathrow Express 'shuttle' or Heathrow Connect services serving Terminal 4.