High-speed trains linking Melbourne and Geelong is the government's new plan for the booming west, despite the multibillion dollar Regional Rail Link opening three years ago.

The Andrews Government will set aside $50 million in the 2018/19 state budget for detailed planning to investigate a fast rail link connecting the two cities, with a business case set to follow.

The trains under consideration would be capable of travelling at speeds of 250 to 300 kilometres an hour.

A map of the proposal shows the Geelong line connecting to Melbourne Airport via a new rail link from Sunshine.