The likelihood of a "faster" passenger train linking Western Australia's capital with its major tourism and food region may hinge on an evaluation of its social, economic and environmental benefits.

The federal and state governments have already committed $8 million toward "high-level" investigations of a Perth to Bunbury service which could operate on a separate track to an existing line which carries a multi-stop commuter diesel train.

That service, the Australind, is set to be compromised for up to 18 months by an upgrade of the Armadale rail line in 2023.

The new line would also have the potential for further transport connections to the Busselton and Margaret River tourist region.

WA Transport Minister Rita Saffioti said a $500,000 planning contract awarded to international advisory service KPMG to have a business case ready by mid-2024.

"The strategic business case is a first step in understanding what a faster public transport link between Perth and Bunbury would look like," Ms Saffioti said.

"The business case will use population data, travel patterns, land use planning and other metrics to asses the project's economic, social and environmental benefits."

But she said if the study found it feasible, it was "realistically many years away".

How fast is fast?

South West Liberal MP Steve Thomas called for clarity on the government's use of the term "faster rail".

"Fast rail is the old proposal which would bring a train line extension from Mandurah to Bunbury or whether they are looking at increasing the speed of the [existing] Australind train service," Mr Thomas said.

"The fast rail proposal is a multi-billion dollar exercise which has been talked about for many years and to this date, the business cases that have been done haven't stacked up.

He said speeding up the Australind was a much cheaper option but wasn't sure if it was the best option.

A state government spokesperson said the $500,000 evaluation would examine a "range of transport links between Perth and Bunbury".

Power to be proven

Bunbury MP Don Punch said dual lines would most suit the region's growing needs.

"The existing south west line has a lot of freight on it and an all-stops commuter train I think is going to continue into the future," Mr Punch said.

"As the economic and social conditions grow, we will see the opportunity for an express service."

Mr Punch said the combined $8 million state and federal funding would also probe what may power the train.

"This particular study will look at the engineering including new technologies," Mr Punch said.

"That might a hydrogen-based train which we are starting to see in Europe ... my sense is it will either be electrification or an alternate fuel."

He said the study would also seek to reserve land for the rail line.

Long history of train talk

The potential for faster rail links between Perth and Bunbury has been mooted for the past two decades.

Former Labor premier Alan Carpenter commissioned a feasibility study in 2008 into the cost of a diesel train which would travel down the Perth to Mandurah line to Cockburn central, then follow the Forrest Highway.

That study found extending the Mandurah rail line to Bunbury would be hugely expensive, costing at least $1.2 billion.

In 2015 the Barnett government paused the project, but it re-emerged as an issue in the lead up to the 2017 election, where the government shifted from Liberal to Labor.

The Nationals pledged $3 million for another feasibility study into the extension of the Mandurah line.

Labor rubbished that plan, and instead campaigned heavily on a $30 million pledge to upgrade the Australind diesel line, which runs through inland towns like Harvey and Brunswick.

In 2017, then Labor candidate Don Punch described it as a "fast-train mirage".

"Why would you spend $3 million replicating work that's already been there?" he told the ABC, five years ago, referring to the previous plan.

But come December 2020, the state and federal governments announced the fresh study at a cost of $8 million.

The plan comes against a backdrop of billions of dollars in cost blowouts for the state government's metropolitan area rail expansion Metro.