The federal government has pledged $5 billion for a Melbourne airport train line and wants the Victorian government to get on board with a 50-50 funding split.
Prime Minister Malcolm Turnbull announced the funds on Thursday, with construction possibly starting "not long after" 2020.
"What we're putting to the state government is we will own and build the rail link together as partners," Mr Turnbull told 3AW radio.
"We'd invest up to $5 billion for a 50 per cent share."
The rest is envisaged to come from the private sector, he said.
Mr Turnbull said while four routes had been canvassed, he did not have a preferred route yet and this needed to be carefully considered.
The project has been discussed for decades, initially by former Victorian Liberal Premier Henry Bolte.
"This is a real game changer," Mr Turnbull said.
"It's been in the too hard basket for far too long.
"Five billion is the type of money and the type of leadership that we need to get this project started."
Mr Turnbull was certain he could reach agreement with the state government to get it done.
He has written to Premier Daniel Andrews suggesting the two governments put in equal funding for the project, possibly using $2 billion Victoria will get from selling it's share of the Snowy Hydro Scheme, News Ltd reports.
Mr Andrews said in 2016 an airport rail wasn't a top priority, but last November he announced works on an Airport Rail Link would be under way before the completion of the Metro Tunnel and would deliver high speed rail to the regions.
In the last budget the state government funded the current business case.
Public Transport Minister Jacinta Allan said Victoria had already committed to an airport rail link and was happy to work with any Commonwealth government on the project.
"It's good the Prime Minister has finally found Victoria on a map, and we'll accept this funding, given how much he short changes our state," Ms Allan said in a statement on Thursday.
"Our plan for airport rail also includes fast rail to the regions, so its concerning that only one of the routes proposed by the Commonwealth would even allow for that."
