Manchester Airport rail station had 3.3million passengers in 2013/14 which will not include MatroLink of course.
Gatwick has 16.2million in the same period
Birmingham International had 4.9million in the same period although clearly it also provides access for other things more than MAN and LGW.
LHR has a range of stations that I can't be bothered looking at and adding up.
So MAN has come a long way from zero rail passengers in 1992 to 3.3million now (doubling from ten years ago as well) but Gatwick shows that many more passengers could travel by rail even taking into account of the different number of passengers using the respective airports.
Fourth platform will help.
The other Northern Hub improvements will help (upping frequency and reducing travel time).
Western access would help if it ever happens.
Overall, 'The Station' is something that MAN can be proud of.