Commuters travelling between Geelong and Melbourne will cut their journey time in half thanks to a $2 billion government commitment for a fast-rail link.

But Prime Minister Scott Morrison concedes such a journey won't occur for years, with construction expected to start within two years after the business case is finalised.

"You can't go through a McDonald's drive-through and get yourself a fast-rail to Melbourne," he told 3AW radio on Friday.

"These are big projects, they can't just be done by yesterday afternoon."

The funding for the 20-year fast rail plan will be included in the April 2 budget.

As part of the plan the federal government is putting $40 million towards developing five new business cases across the country for the fast-rail links, which will travel at an average speed of 160km/h.

Such a speed will see the 80km Melbourne to Geelong commute cut in half to 32 minutes.

The Melbourne to Geelong route is set to be one of many built across the country over the next two decades.

"As our population grows, fast-rail networks are crucial to easing the congestion pressures in our cities and shaping Australia's future," Mr Morrison said.

The rail projects will help ease pressure on major cities and make regional areas more attractive, he added.

The fast-rail announcement is the latest slice of the government's population growth plan, with Mr Morrison announcing earlier this week the number of annually available immigration places will be cut from 190,000 to 160,000.

OTHER RAIL LINKS IN THE WORKS

* Brisbane to the Gold Coast

* Melbourne to Albury-Wodonga

* Melbourne to Traralgon

* Sydney to Wollongong

* Sydney to Parkes