In fact, it's normal to change the destination at (or just after departing
from) Hatton Cross, so that people waiting at T4 and wishing to get into
town don't get confused by a train turning up with "Heathrow" on the front
(at least, that's the way I used to do it). The operational switch from WB
to EB takes place at Heathrow 123.
--

Cheers Steve - Hatton X WB would seem logical, but how many people board at
T4 for T123, especially when the HX is free between these two points?

Regards

Matt