Most journeys are not 'city centre to city centre' just like most journeys are not 'airport to airport'. It is thus iniquitous to compare rail and air other than on a strictly case-by-case basis. The key thing is that airports and railway stations should be integrated into a transport system that makes changing from one 'mode' to another is simple and straightforward.
I'm sure we can all think of good and bad examples of this:
Good: Southampton - on-site railway station and motorway nearby;
Bad: Teesside - infrequent buses and undeveloped rail potential.