Hop on the Piccadilly Line in Central London and in 50 minutes you can debark at any of the terminals at Heathrow Airport, without changing trains. Get on BART in downtown San Francisco, and one can go directly to the perimeter of the International Terminal. Getting further into the airport requires a transfer to SFO's "Airtrain." That's similar to the way things work at Oakland, New York, or many other American airports, where transfers between urban rail systems and people movers" are typical.