Would the obvious solution to expansion packs being slow to start selling not be to release them 6 months or a year after the starter sets?
I also imagine that the reason that the train lines don't sell as fast is the high upfront cost to them. Parents may baulk at forking out £100+ on one Christmas present but may be more willing to part with their money if they could do so one step at a time. I.e. just buy a locomotive (perhaps with the PF kit sold separately as per the Creator Expert Fairground to split off another of the upfront costs), Granny could buy a wagon, Aunty Mabel could buy a track pack. It doesn't even have to be all over the same birthday/Christmas.
On the original topic, I'd far rather see the window packs re-released or at least available on the S@H PAB site. They would obviously need a set to warrant firing up the production lines in the first place so something that uses them in tan (just about the rarest, most desirable colour). Failing that, I'll just have to put any GWR coaches to one side and concentrate on those colours I can get hold of. I may just have to do a Royal Train to find a use for all those dark purple ones there are on Bricklink.