wrote

What size market exists for travellers between Heathrow and Gatwick I
wonder? There are frequent coach links, until fairly recently (early
1990s?) a roughly 90 minute frequency S61 Helicopter service, and in
the 1960s and 1970s Westward Airways had a BN Islander aircraft passing
over our house almost hourly. It would be good to take some of those
coaches off the M25/M23.

It would, but it would be well nigh impossible to devise a fast rail route
on which robust paths could be provided -
Airtrack to Clapham Junction, but then either reverse at Stewarts Lane, or
negotiate Factory Junction, Herne Hill, Tulse Hill to get to the Brighton
Line
or GWML, WLL, and Brighton Line, all three of which are congested, and the
link via Acton Wells and Willesden Junction is slow (I suppose when E*s go
it would not be impossible to reopen the Old Oak Common East to North Pole
spur)
or resurrect the idea of going via Dudding Hill on to the MML then via
Thameslink.
I suppose if the Central Railway ever gets built it would not be completely
impossible to construct a few spurs and beg some paths between the freights.

Peter