Why it should be there: the very reason of having a subway is saving space on the surface. Upgrading entrances should be preferrable to building more of them.
What will it do: Upgrading an entrance adds platforms to the subway station or enlarges them. This will lead to the station being able to deal with more passengers as both arriving and leaving
How can it be achieved: Click on an entrance and open an upgrade interface like stands have. Select upgrade to X platforms for Y money. This will change the number of PAX it can “digest” from airport as well as PAX arriving there by train.
Reason for suggestion: Most capital buildings and even airports do not have more than 2 subway stations and those entrances are usually connected underground. The current way things are done forces us to build more of these items because taxis and buses cannot cope with large airport operations PAX-wise. Queues get too long.
Suggested solutions are:
a) build more entrances
b) “simulate” that subways can be expanded to accommodate larger passenger numbers
I choose b) over a) as it is the more realistical approach.