Roland Perry wrote:
In message
-sept
ember.org, at 09:11:07 on Mon, 2 May 2016, Recliner
remarked:
Don't they go all the way through to Uxbridge in the peaks?

No, only about half do. See my other post:
"I just looked at the Rayner's Lane departure board, and in the next 15
minutes there will be six through trains (five Met, one Picc) and two
terminators."

Are they running a weekday, or a Saturday, service?

I'm not sure. But, either way, it proves it can be done, and happens
routinely.

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. Why would it affect the existing platforms?

Note that the Piccadilly line already has reversing sidings beyond the
platforms. And something else that hasn't been mentioned in this thread is
that there have been regular calls for the Piccadilly line also to be
extended to the west.