Will start in 2016 and open in 2021 - spur of the main line ant Langley allowing trains from Reading and apparently Bristol & S Wales direct access
Quite why it wasn't built 10 years ago no-one knows
By then the lines from Paddington to both Bristol Temple Meads via Bath and to South Wales via Bristol Parkway will be electrified, or should be - nothing can be taken as read with any government of any political colour - but the entire South West England west of Bristol will carry on in the diesel age.
Part of the electrification rationale was a speeded-up service to that currently provided by the 40-year old First Great Western HST sets. It's not made clear how passengers will access LHR via the proposed spur. It would be nonsensical for the mainline trains themselves to divert in and out because the overall journey to London itself would then be no quicker than it is now.
Presumably passengers will change trains at Reading but the LHR press release is short on detail anyway although the notes to editors might give a clue. It goes into specifics about how much time will be saved on journeys from South Wales stations but says nothing about those from the West Country and South West, perhaps because the latter will remain firmly entrenched in the railway 20th Century.
Heathrow - Press releases - Heathrow western rail access to benefit millions