What discussions it has had with Network Rail regarding upgrading the railway station at Glasgow Prestwick Airport.

I can advise that the matter of potentially upgrading the railway station at Glasgow Prestwick Airport remains the responsibility of the owner, Glasgow Prestwick Airport.
Glasgow Prestwick Airport operates on a wholly commercial basis, and at arm’s length from the Scottish Government. The airport is very aware that the current condition of the train station would benefit from significant investment befitting an international gateway.
However, this needs to be considered alongside all long-term investment needs for the airport, to generate a long-term return for taxpayers.