I don't get this obsession with a railway station having to be built at the airport. As has been mentioned already, EMA, LPL, LTN, BRS, GLA, EDI, etc, all seem to manage quite nicely with a Parkway concept.
When I lived in both the east and west midlands BHX became my, very much so, preferred airport of travel because of the Birmingham International train station which, I hasten to add, isn't actually at the airport
I would park my car for free at a train station one stop down the line and take the train in/out of the airport ... it worked well.