What Toronto has done, with a dedicated express to the airport on the outskirts to the downtown hub rail station, is probably a sensible infrastructure pattern for many places. But it was a happy coincidence an early 20th century line ran most of the way there already. Otherwise, it would have meant laying a line through upper middle class suburbs. That's a showstopper, politically, whatever the merit might be; rail lines almost never get built in areas that are already heavily developed.