The proposed HS2 high speed line to Birmingham would be in a tunnel for most of its journey through Central London and much of the Chilterns in Buckinghamshire, but on the surface as it passes through North Acton, Perivale, South Ruislip and West Ruislip, for much of the way running beside the existing Metropolitan Line Uxbridge branch. There’s a local opposition group in Hillingdon.
Its main hub station would be on the old Great Western sidings at Old Oak Common, connecting with Crossrail and the Paddington line. This is where Brunel’s London to Bristol railway line comes to within a few hundred yards from Stephenson’s slightly older London-Brimingham line (the reason is that before Paddington was built the Great Western planned to share Euston station). Here are the plans prepared for the Department of Transport for a new station at Old Oak Common; and here is a supporting presentation by Hammersmith and Fulham council. The alternative plan drawn up by Arup Associates for the hub to be located instead at Heathrow Airport (apparently on the site of existing reservoirs in the Colne Valley) has gone nowhere for the time being — perhaps because the long term goal is airport expansion elsewhere.