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?
--
Roland Perry