Transport Minister Troy Buswell says Labor's Metronet rail plan needs a further $1.1 billion to duplicate the Midland line between Bayswater and Perth.
But he has so far refused to release details of work he commissioned from the Public Transport Authority on which he bases the claim.
With the row over rail costings continuing to dominate the election campaign, Mr Buswell called a news conference yesterday in a bid to firm up his claim that Labor's ambitious rail plan would cost $6.4 billion as opposed to Labor's $3.8 billion price tag.
Metronet includes an Ellenbrook line, airport spur and northern and southern circle lines.
The plan is to connect the Ellenbrook line, airport spur and northern circle line to the existing Midland line at Bayswater station.
Mr Buswell said PTA advice commissioned before the Government's caretaker period began last Wednesday, but not verified by Treasury, was that the Midland line could not cope with the increased services at peak times.
It would need to be duplicated between Bayswater and Perth stations, including a tunnel between East Perth and Perth stations.
"The Labor rail plan is unaffordable, it is unachievable and it will not happen," Mr Buswell said.
Shadow transport minister Ken Travers rubbished this claim, saying the Midland line could handle 20 trains an hour based on information in the Government's own public transport project definition plan prepared for the Perth major stadium project.
He said Labor planned to run 18 trains an hour between Bayswater and Perth with Metronet - six on the Midland line and four each on the new Metronet lines.
"Mr Buswell's claim that under Metronet the capacity of the Midland line will be stretched beyond its limits and would require a duplicate rail line is completely false," Mr Travers said.
Labor has refused to have Treasury cost its Metronet plan and Mr Buswell has made claims about a range of Metronet projects but has not substantiated them with the PTA costings on which he says they are based.
The PTA was asked to release the costings but a spokesman said last night it would not.