It's used almost exclusively on the Heathrow Express shuttle service from Terminals 1/2 to Terminal 4
They must have to take some serious precauations to stop the driver on the T123 to T4 shuttle going absolutely stir-crazy!
Heathrow Connect services
PA▸ currently announces this station as something like "Heathrow Terminals 1 and 3" or "Heathrow Central for Terminals 1 and 3". Can't remember which one they use but this is because Terminal 2 closed a year or 2 ago, has been demolished, but will be rebuilt as a larger terminal. Terminal 1 will eventually close. Can't remember if T2 will expand into where T1 is now or if it just takes over the existing T1 piers and stands.