Victorians will have to wait decades to see the fruits of Labor's $50 billion election pledge for an underground rail loop.

Premier Daniel Andrews on Tuesday unveiled the Suburban Rail Loop – hailed the biggest transport project in the nation – which would connect train lines in Melbourne’s suburbs.

The route would link up to a number of suburbs including Clayton, Glen Waverley, Doncaster and Box Hill, and also link up to Tullamarine airport and finish at Werribee.

Construction would start by the end of 2022 and finish by 2051.

“I won’t be the premier when this project is finished. It if I am re-elected I will be the premier who gets this project started,” he told reporters on Tuesday.

“It will take time and it will represent very, very significant expenditure. But what’s the cost if we don’t build the Suburban Rail Loop – more congestion, and a city as it continues to grow that just won’t function.”

Under the plan, up to 12 new underground stations would be built.

The state government will set aside $300 million in next year’s budget if Labor wins the November state election for a full business case, design and pre-construction works.

Exact locations, route alignment, rolling stock and staging will be confirmed in the business case and technical work.

The state government would also seek funding from the Commonwealth and private enterprise, and a dedicated authority may need to be set up to deliver the project.

Public Transport Minister Jacinta Allan said it would be “the busiest train line once completed” and would transform how we move around.

The state government says it has been working on the project for 12 months with Development Victoria, Pricewaterhouse Coopers and the Office of the Coordinator General.

Yet Infrastructure Victoria is understood to not have previously given recommendations on the project.

The underground southeast Cheltenham to Box Hill section will be the first underground section to be built and will include six new stations.

The Airport Rail Link would form the northwest section of the loop, with construction slated for 2022.

About 400,000 passengers are expected to use the underground loop daily.

Labor leader Bill Shorten told ABC radio on Tuesday the numbers would have to be studied, but Mr Andrews was “on the right track”.

However, Opposition Treasurer Michael O’Brien said the proposal should be sent to Infrastructure Victoria for proper assessment, saying there was no detail on its cost or how it will be funded.

“With some estimates suggesting it will cost between $50 billion to $100 billion and a completion date beyond the year 2050, this looks more like a plan for the next election rather than a plan for the next generation,” he said.

By Christine McGinn