In message
-sept
ember.org, at 10:02:23 on Mon, 2 May 2016, Recliner
remarked:

One thing that's not been mentioned yet is that this Heathrow reversing
siding would have to be newly constructed. Why do that when there's
already a pair of platforms installed and ready for the through-running
trains?

Well, the through lines also have to be constructed to the west, so
building a reversing siding (or even a pair of sidings) between them at the
same time would be trivial.

I'll give you that one.
--
Roland Perry