Melbourne's southeast metropolitan train network will get a $450 million extension if the Liberals have success at November's state election.
Victoria's opposition on Tuesday committed to matching $225 million in federal coalition funding earmarked for electrification and duplication of the Frankston line to Baxter.
The announcement by Prime Minister Malcolm Turnbull and Liberal leader Matthew Guy came as the state Labor government continues to compile a business case for the project.
"It is vital to improve mass transit across the city and across the state," Mr Turnbull said.
Baxter is currently serviced by diesel trains on an hourly basis and is under-utilised by travellers.
"Part of the problem is that the service just doesn't incentivise people wanting to get it," Mr Guy said.
"We could have a real park-and-ride system or a drop-off-and-go system."
Under the project, the Frankston train line would be extended by eight kilometres, there would be new stations at Frankston East near Monash University and Frankston Hospital and at Langwarrin, and an upgrade of the Baxter train station.
"That business case won't be ready until next year," Premier Daniel Andrews said on Tuesday.
"Like so many projects, we are doing the planning, the careful and detailed planning so that you can get to the point of making commitments."
The Victorian and federal governments are also working on a business case for a train line to Melbourne Airport.
"The rail to Melbourne's airport, to Tullamarine, is long overdue," Mr Turnbull said.
Mr Andrews said his "productive" meeting with the prime minister on Monday focused on the airport rail project, not the Frankston line extension.
