In message , Spyke
writes
Surely once you're running trains to Maidenhead, it would make sense to
extend them a couple of stops to Reading, where there's a far greater
range of connections and plenty of bay platforms to terminate them.

The problem is the extra cost of the electrification. Presumably it's
not regarded as cost effective to extend it from the Heathrow branch to
Maidenhead *and* beyond to Reading (although I'm sure the very first
Crossrail proposals *did* have that as an option).
--
Roland Perry