A massive, $50 billion, underground suburban rail network will be built, linking every major rail line in Melbourne and the new airport rail, the Victorian government has pledged.

Billed as "the biggest public transport project in Australian history", the Suburban Rail Loop is expected to take almost 30 years to build and connect key train lines from Frankston to Werribee.

When completed commuters would not need to travel into the city and back out to reach another part of Melbourne – a major downfall in the city's existing train network.

The 90 kilometres of track would connect 10 train lines and the entire project is expected to take until 2050 to complete.

The line would be fully underground from Cheltenham to Melbourne Airport, and partially underground between the Airport and Werribee.

Premier Daniel Andrews said the project was about connecting people to jobs more easily.

‘‘This is about connecting universities and TAFEs and hospitals and employment precincts, it’s also about being able to live closer to where you work," Mr Andrews said on Tuesday morning.

"This is about taking hundreds of thousands of cars off the road and giving people a public transport system that they can leave the car at home for.

‘‘It will take time to build, it will take a lot of money, but the real question is what’s the cost if we don’t deliver this? A congested city that just doesn’t function.’’

The new line would connect every major train line and will start to shift the rail network away from being a purely radial layout to an orbital design - one that includes circular links around the city, and not just in and out of the CBD.

It would be built in stages over multiple decades and is designed to connect major employment, health services and education precincts.

Business case development and the detailed design would start next year to determine exact station locations, route alignment and rolling stock.

The loop would connect 15 stations in total, with interchanges built at 10 existing stations.

Five new stations would be built at Doncaster, Bundoora, Burwood, Monash and the Airport, but commuters would not have a direct link to the city from these sites.

Work on a 25-kilometre section from Cheltenham to Box Hill will begin by 2022 and is expected to take about a decade to complete. Construction on the section between the Airport and Sunshine would also start in 2022.

Construction of the new stations would start near Cheltenham. Existing stations including Glen Waverley and Box Hill would would be upgraded and may need to be rebuilt to link up to the underground service.

Heading north, the new line would travel underground, with potential connections at Doncaster, Heidelberg, Bundoora, Reservoir, Fawkner and Broadmeadows.

It would then join with the new airport rail link and Sunshine and Werribee.

Regional rail interchanges would be built at Sunshine, Broadmeadows and Clayton.

Building the rail link would be highly disruptive. As with the Metro Tunnel project, massive tunnel boring machines will be used to dig underground from portals at different points of the tunnel.

The rail project was never considered or recommended by Infrastructure Victoria, which has pushed for a different underground rail link connecting Newport and Clifton Hill, via Fishermans Bend.

Premier Andrews said he expected there would be a private sector "stampede" to get involved in the project.

"We will announce all of our costings, and all of the different elements of our complete package ... before November 24."

Opposition treasury spokesman Michael O'Brien said the proposal should be sent to Infrastructure Victoria, which provides independent advice for the government, for assessment and planning.

He said the government could not say how the project would be funded.

"They have no business case, no engineers' report and they won’t rule out more sky rail across Melbourne," he said. "They refuse to say how many thousands of homes would be compulsorily acquired."

Mr O'Brien accused the government of putting up a "plan for the next election rather than a plan for the next generation".

A spokeswoman for Infrastructure Victoria confirmed the advisory body had not seen any details of the proposal.

She said the body's lack of knowledge of the plan proved Infrastructure Victoria was independent of government decision making.

Mr Andrews told Triple M radio the government had been working on what he termed the "ring rail" for the past year.

"[This] will deal with congestion," he said.

"Our train system is built at the moment based on everyone going to and from the city and that's just not the way our city and suburbs work.

"This is the sort of rail system that all the great cities of the world are building."

He pledged to start the project if re-elected in November. The project would deliver a "long-term pipeline of work", which is what the construction industry has been asking for, he said.

Public Transport Minister Jacinta Allan told 3AW the government would look for contributions from the Commonwealth and the private sector and expected the project to be finished by 2050.

"This is a big investment, I will certainly acknowledge that. The estimates that we have it will cost tens of billions, potentially up to $50 billion," she said.

"They have no business case, no engineers' report and they won’t rule out more sky rail across Melbourne,"