Smacks of desperation from The Chambers of Commerce to me - and my guess is that DfT have floated it just to let it get shot down.
It's absolute nonsense of course - a mainly underground high speed line is going to cost how much? Where is the money to come from? How long will it take to build even if it did get planning permission? And what will it achieve? B*gger all I would suggest
It will of course only take 15 mins to make the journey - but if you are transferring, you have to get off the plane, get to the rail point, wait for a train, get to the other airport and transit through to your gate. Total gate to gate time much longer.
And it doesn't increase the runway capacity, so what's the point? The opportunity for the UK to retain its pre-eminent role in World Aviation has gone now due to Government dilly-dallying for the last few decades.
Other countries and airlines have spotted the opportunity and grabbed it.
The only thing this idea is likely to do is decrease the capacity of the London TMA due to the increased numbers of pies and pigs flying around until it is ruled out.
Suzeman