In message , at
05:59:21 on Wed, 24 May 2017, Ding Bat
remarked:
If construction is allowed in this pipe-dream, then the plan is to
extend the line through Terminal 5 towards Slough.

Ah, so there's such a thing already in the works! Thanks for the information.
The underground portion of that line will be from T5 to Langley, according
to this:
http://www.bbc.com/news/uk-england-berkshire-35803950

Projects like that are a minimum of five years late, so if it was
suggested they might start tunnelling soon, don't hold your breath until
2016 + 5 years work + 5 years standard delay for an actual service.

Has it even been approved yet (genuine question).
--
Roland Perry