The Doncaster rail line – all but promised by the Coalition before the 2010 election – is another 15 years away from being built, a Napthine government study has found.

Government authority Public Transport Victoria also predicts the railway line would prompt just 2 per cent of motorists to switch from cars to trains, even though the line would service one of the most car-dependent parts of Melbourne.

About 56,000 passengers a day would use the line by 2031, PTV predicts.

In the meantime, work will be done to give residents of Melbourne's north-east better bus services in and out of the city.

The authority also said it would investigate ways to build the Doncaster rail line for less than the $3 billion to $5 billion suggested in a feasibility study.