Never going to happen and it has been well chewed over in the past.
- The Nimby's of Northolt are well healed and if you think the citizens of Sipson whipped up a storm - wait to see this lot!
- As Skipness says, folks have no idea about how much time to leave getting across one terminal, leave alone a T1-T4/5 connection. Asking them to take a fast train to Terminal N, that takes a bit longer? Nah. They will belly ache all day long and spread very bad publicity for EGLL.
It's a pity about the above, because I thought it was the best idea when I first heard of it over five years ago. It makes sense for many reasons.
I can only repeat, no new runway will be built in the South East of England in the next 15 years and probably longer.