Regarding train connections to the airports.
Quite often I see the trains from Brighton which run up from Clapham Junction to Willesden and Watford (they used to go on to Milton Keynes). They normally have no more passengers on board by the time they get to Watford than would fit in a taxi.
You can also see the Heathrow Connect, which is useful from Heathrow in as far as Ealing but then just duplicates other services on the last lap to Paddington.
A connection between these two, for which the groundworks are pretty much there anyway, would seem to offer a whole range of new opportunities, as well as eliminating some unnecessary train operation. From Brighton and Gatwick through Croydon, Clapham, Kensington, Shepherds Bush, Ealing, and on to Heathrow. This opens up all sorts of useful links like Ealing to Gatwick or Croydon to Heathrow. It is an efficient use of the train, because as those from Gatwick are getting off, those for Heathrow are getting on.
I guess however it is the sort of scheme which will not see the light of day because it does not involve big investment in civil engineering or new trains, so the major contractors and manufacturers who push such schemes along would not be interested.