The government have got themselves into a big hole, realising that the third Heathrow runway is the only sensible national approach. Instead, a combination of ever-increasing APD on domestic flights, and remaining Heathrow domestic slots being flogged off to the highest bidder (mentioning no names, BMI) is turning much of the country into having their main air transport option being as a remote spoke to Amsterdam, Frankfurt, etc. It's not as if the domestic flights were not well used, and in fact from my experience MPs themselves always seem to make up a notable proportion of the total ! Notice how Gordon Brown (MP in Scotland, a BA regular on LHR/LCY-EDI) never squashed the proposals like David Cameron (lives in West London, MP for Oxfordshire, never flies from a provincial UK airport) has done.
If only BAA had presented the closure of runway 23 some years ago and the new building of the north runway as one "runway realignment" project, I feel all the opprobrium associated with the words "new runway" could have been avoided. Airport moves from having three runways to having - three runways. But BAA
PR were asleep at the time.
The rail link is a ludicrous proposal, it seems nobody at the DfT has noticed that the airports of London have stratified into different airports serving different carriers and market types. There is actually very little connecting traffic between the two at present, and not much future potential (DfT does not understand/hates connecting traffic anyway, and has gone well out of its way to put more hurdles in the way of connecting passengers at Heathrow than any other government elsewhere has done). Huge cost for the rail link, little demand. A typical government idea. As it would also involve far more digging up alongside homes in marginal political constituencies along the way than the third Heathrow runway would ever need, it's a complete non starter.
I have always thought there is a much more straightforward approach for those smaller numbers who do go between the airports. The Heathrow Express provided a direct train to Paddington. The Heathrow Connect serves stations along the way, so does not need to go right through to Paddington as well. It passes beneath the railway line that takes trains from Watford down the west side of London, through Kensington and Clapham Junction to Gatwick, which itself carries few passengers at the Watford end. Just linking these two with a straightforward bridge (all the land required is already in railway ownership) would enable trains to run regularly from Heathrow to Gatwick via these points. No extra train mileage required at all. If it ran every half an hour you would might get about one train coachload of passengers between the end points, but it would also suit nicely Clapham or Croydon to Heathrow, Ealing to Gatwick, and a range of other journeys all mixed together. Which is what trains are good at.