In the early 70's at the behest of airlines Congress passed a law that prevented Airports and State/Local governments from sharing cost of Municipal Rail lines.

In the early 70's at the behest of airlines Congress passed a law that prevented Airports and State/Local governments from sharing cost of Municipal Rail lines. Which is why in the US if there is a rail connection it's always a separate feeder. I only learned of that a few years ago when congress repealed it. There is also an unwritten law in the US that the government isn't allowed to make changes that would interfere with an established business charging monopoly rents. Putting a subway stop at an airport would devastate the business of taxi, limo, and parking concessionaires. Also can you imagine if you had a layover at an airport and could get on the subway, go downtown and have dinner, then come back?