Pune is not very far from Mumbai and there are reasonably good rail links between the cities , but it is not really close enough to be considered as an airport to serve the Greater Mumbai area.
In any case, I can't see too many business class passengers wanting to travel on Indian trains!
Indian trains are fine if you've got a reservation (which wouldn't be much use of course if your flight was late), but even with a reservation, they're not exactly up to business class standards of luxury.