Scott Morrison's promise to build 200km/h fast rail services between Melbourne and Geelong will not be possible without new trains, a leading expert says.

The Prime Minister has promised to build high-speed rail line where trains would travel up to 200km/h, slashing the travel time between Geelong and Southern Cross Station to 32 minutes.

It would take about 10 years to build, with construction to begin in two years and the state government expected to pitch in $2 billion.

But Mr Morrison's $4 billion pre-election cash splash does not include money for new trains.