What you might call phase 1, some sort of people-mover between
Brent Cross Northern Line and (proposed) Brent Cross Thameslink,
looks achievable and could help reduce car journeys in the area.

Some kind of shuttle bus is planned. I seem to recall a suggestion
that it be like the things you find in tourist areas in the USA,
designed to look like an old fashioned tram, rather than a bus.

A few years ago there was a suggestion to use the Dudding Hill line
for a branch of the Heathrow Express, running in to St Pancras, but
that's dead now, I think

Jeremy Parker