An Airport+Station is essential, but not all major airports have gotten around to building them.
https://en.wikipedia.org/wiki/International_Airport_railway_station,_Brisbane
https://en.wikipedia.org/wiki/London#Rail
https://en.wikipedia.org/wiki/Charles_de_Gaulle_Airport#RER
https://en.wikipedia.org/wiki/Glasgow_Airport_Rail_Link , https://en.wikipedia.org/wiki/Glasgow_Airport_Rail_Link#Planned_reinstatement_of_the_link
https://en.wikipedia.org/wiki/San_Francisco_International_Airport#Transit
