The construction of an airport rail link in Melbourne is a step closer after the federal government backed the proposed route supported by the state government.

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.