The Morrison government is considering ditching an airport rail tunnel in favour of a state government plan for an above-ground track in order to get the multibillion-dollar project done.

Sources close to the project said the Morrison government still wants to build a tunnel to Sunshine as part of the city's new airport rail link but is open to the state government's plan for a cheaper option.

Prime Minister Scott Morrison and Victorian Premier Daniel Andrews speak to media about the airport rail link at Sunshine train station. Credit:Stefan Postles

Under the Andrews government's alternative proposal, airport trains would run along a new surface-level branch line between Tullamarine Airport and Sunshine, before moving onto existing tracks into the city, via the new Metro Tunnel.

The Rail Futures Institute has warned that without a new six-kilometre tunnel, airport trains would be jammed in with metro and regional trains on congested tracks and plans for high-speed regional services would be delayed.