IT IS the dream that will not die: Melbourne to Sydney by train in four hours, instead of the current 11½ hours.

Today, Greens Leader Bob Brown will launch a campaign at Southern Cross Station for a major concept study into a high-speed rail link between Australia's two biggest cities, which his office has costed at $40 billion.

This follows the launch in January of a ''pre-feasibility'' study by the Cooperative Research Centre for Rail Innovation, which examined a plan to cover the 900-kilometre Sydney-Melbourne route by train at an average 280km/h.

Mr Brown said a high-speed rail link was needed to provide ''fast, reliable transport for 75 per cent of our population … [And it] would also generate thousands of jobs and promote regional development.''

He called on the Rudd government to fund a $10 million one-year planning study.