The airport is at the top of a hill and the railway is at the bottom so a station by the existing terminal is impossible. There was a detailed plan drawn up to site the main terminal by the Parkway Station and have a Gatwick style monorail up to the existing terminal which would become pure airside. In fact the tunnel under taxiway Alpha was built so a monorail could be built on the other side of the concrete pillars. It might still happen if the airport can buy enough land from Vauxhall motors.