happy,
Sorry to be the one to pull this thread back on track (pardon the pun) but rail links, important as they are can be changed to suit any location
They need to be thought through just as much as the issues you mention - but I don't see a new town of that size being built either, so fast links need to be built throughout the London area, and beyond, so that workers and passengers alike can easily get to and from the new site.
Crossrail can provide a
regional stopping service - but to compensate for the displacement such an airport would cause, there needs to be
something much faster for people living north and west of London, and there needs to be a non-stop (or 1 at max) service to central London.
The logical place for such a station would be in what I call the
SPEKX complex - and yes,
you do need to focus now to get this right! Plans for Euston at the moment are for another stub, and an uncosted link between HS1 & 2, which
may handle a train or two per day from north of London into the Chunnel under current plans.
A much better option would be to re-think the Euston design - turn it to face east-west, and use the airport or Ashford as a terminus for HS2, with many more Eurostar / DB etc trains then being able to run north as they are able to set down
and load again in this station.
I still don't think this airport will happen, but there is no doubt that this government is
very much more serious about it than the last one was about Cliffe.
For these reasons, planning to get rail access right needs to be considered
before the HS2 hybrid bill goes through parliament.
If, on the other hand, LHR gets its 3rd, then one planning condition maybe improved surface access, including a spur from HS2 - but that would not have a major impact on the layout of the planned stations. As for LGW, they should start thinking about a north-south continuation of HS2 for that, again needing a re-think of Euston's stub platforms - and a runway at STN would also need a high speed link, probably from HS1 at Stratford, so that would also need a terminus or a continuation onto HS2.
Airport access isn't the primary goal of HS2, but it would be much better to consider these options now, rather than to pay a fortune to adapt lines / stations for this extra traffic (if you'll excuse my pun now) further down the line.