The Sunshine route, preferred by the Andrews government, would run fast trains from Southern Cross station to Tullamarine through the western suburbs with the potential to link Geelong, Ballarat and even Bendigo into the network.

The government said last month a tunnel between the city and Sunshine could also be built as part of the Andrews government's plan for an airport rail link.

It's understood federal Infrastructure Minister Alan Tudge wrote to the state government weeks ago to confirm he was supportive of the Sunshine route.

But Public Transport Minister Jacinta Allan said while she welcomed the federal government confirming its support for the project, the route was locked in by Victoria months ago.

"The Sunshine route allows us to deliver an airport rail link that benefits all Victorians," she said.

Opposition leader Matthew Guy said in July the Sunshine route was the obvious "common sense" option.

“That was really the only sensible route," he said.

“The upgrade of that line, connecting back to the airport was the most sensible, obvious, straightforward way to do it. It's amazing it's taken the government nearly four years to come to that realisation."

The huge infrastructure project was kick-started by a pledge of $5 billion from the Turnbull government's federal budget in May.

Mr Andrews and Ms Allan have promised to match the Commonwealth investment with $5 billion of state money to build the railway if Labor retains government at November’s state election. It's estimated the project will cost between $8 billion and $13 billion.

Other options which have been previously explored include a route through Highpoint and Maribyrnong, creating a new suburban line and opening up new growth corridors in Melbourne’s booming north-west, as well as a third suggestion using the Craigieburn line with a spur between Essendon and Tullamarine.

The Australian Pacific Airports Corporation has also back the Sunshine route, saying a line from Southern Cross to the airport with a major interchange at Sunshine is the “clear front-running route” for the project.

The Andrews government said it would release a preliminary business case for the airport rail link in September, but the document has not been made public.