I had a good laugh when i first heard this suggested, but perhaps
there's another way to look at it....

The BML needs relieving with a lengthy tunnel being suggested,
dedicated services to both Heathrow and Gatwick are under threat of
being at least partially absorbed into Crossrail/Southern services,
and the cancellation of Airtrack continues to leave Heathrow without
rail access from the South.

Throwing caution to the wind, might a high capacity 'Thameslink2/
Airport Express' from Brighton/Gatwick to Stansted (or elsewhere north
of London) via a SWML interchange (Surbiton?), Heathrow and Central
London be a [slightly] more practical idea?

Chris