The private consortium wanting to build the airport rail project would charge the state government a negotiated fee every time a Metro or V/Line train is run through the new tunnel.

The toll would be charged in exchange for building, operating and maintaining the twin 7-kilometre tunnels.

Prime Minister Scott Morrison and Premier Daniel Andrews outline options for an airport rail link earlier this year. Credit:AAP

In response to questions about its market-led proposal, an AirRail spokesman confirmed the tunnel would have capacity to run up to eight airport trains per hour.

In addition to these services, the state government could run 14 new non-airport Metro or V/Line services per hour through the tunnel, but would have to pay up every time.