Always seems unfortunate the railway never got organised to link Gatwick and Heathrow. There's a train from Gatwick to Kensington Olympia and on to Watford that is invariably empty north of Kensington, while the Heathrow Connect train from Heathrow serves the local places as far as Ealing but then just duplicates the Heathrow Express on into Paddington. Where these two routes cross by the now abandoned Eurostar train depot there is plenty of railway-owned space for a link. Overall journey time stopping at just the main intermediate places would be less than an hour, and more reliable than the M25.
While the numbers of passengers per hour just between the two airports do not justify a frequent train, combining it in this way makes all sorts of other links between south and west London and the two airports possible, Croydon to Heathrow or Ealing to Gatwick, and more efficient than what is in place currently. It could even be called "Airlink" !