There was just a small handful of passengers on hand to greet WA Premier Mark McGowan when he arrived on an empty train at Thornlie station to talk up his Metronet plans to combat congestion in the city.

But the Government insists there will be enough passengers to justify a new line to Thornlie, linking the suburb with Cockburn, when it commences operations in 2021.

Estimates used by the Government for the $474 million project's business case predict that in 2021, a combined 3,100 people will board trains at new stations at Nicholson Road and Ranford Road each day.

Forecast population growth in the surrounding south-eastern suburbs is expected to then boost passenger numbers over the following decade, leading Mr McGowan to insist the project stacked up.

Mr McGowan aims to get more people catching Perth trains.

"We want more people catching rail," Mr McGowan said.

"If we have a circular link in the southern suburbs through to Cockburn … we will see a massive boost in passenger numbers."

Federal funding expected

The Cockburn-to-Thornlie line is intended to let Mandurah Line passengers get to the new Perth Stadium more quickly, while also improving public transport access in the suburbs.

The State Government has now submitted the business cases for the Cockburn-to-Thornlie project and the extension of the Joondalup line to Yanchep, in its effort to win federal funding.

Earlier this year, the Commonwealth pledged to redirect funding from the abandoned Perth Freight Link to those two projects — subject to viable business cases being presented.

Transport Minister Rita Saffioti said that requirement had now been met and federal funding should be locked in.

"We are pretty confident that Malcolm Turnbull will keep to his word and deliver federal funding for these two projects," Ms Saffioti said.

"We want to get east-west connectivity happening so people can move across the suburbs."

Construction of the Cockburn-to-Thornlie is expected to commence in 2020 and be finished a year later, while the $386 million Yanchep project should also be completed by 2021.

The McGowan Government also plans to complete an $863 million Ellenbrook train service by 2022 and extend the Armadale line to Byford by 2023, at a cost of $291 million.