The Morrison government is ramping up pressure on Premier Daniel Andrews to sign up to a $15 billion airport rail tunnel, but the Victorian government is still weighing up its options.

A private consortium, AirRail, is offering $5 billion to build airport rail, which would see dedicated tracks built from the CBD to the airport, via a new rail tunnel to Sunshine. As part of the offer, the consortium would own and operate the tunnel.

Senior figures in the Andrews government are understood to be wary of committing to the tunnel option, with misgivings over the wisdom of having a key piece of Victoria's rail network controlled by private players who would have enormous leverage over future developments of the system.

Scott Morrison's government is keen to sign up for an airport rail tunnel. Credit:AAP

A cheaper above-ground alternative is being considered by the Andrews government.