The first stage of Melbourne's Suburban Rail Loop should be completed even if the rest of the project is eventually scrapped, the state government says.
The 90km orbital rail line was a 2018 election promise of the Andrews Government, with work already underway on the first 26km section between Cheltenham and Box Hill.
The state's independent Parliamentary Budget Office this week revealed building the loop's first two sections would set taxpayers back $125 billion or almost double what the government estimated.
The Victorian opposition said it would scrap the project altogether if elected in November and instead put the money towards the state's overrun health system.
But Victorian Transport Infrastructure Minister Jacinta Allan said the loop's first section should be completed, telling The Age the eastern section alone would provide "enormous benefits" to the state.
"It would be up to a future government to explain why they walk away from it, if they chose to," Ms Allan told the newspaper on Saturday.
Opposition treasury spokesman David Davis accused the state government of being unable to manage money, saying it was clear the Suburban Rail Loop would be a cost blowout.
"A loop is not a loop if it isn't a loop," Mr Davis told reporters on Saturday.
"They said it was going to be a loop at $50 billion. Now we know the two parts are going to cost $200 billion.
"You can do the arithmetic and work out what it might cost if you did the third part too.
"None of us know and the government won't tell us."
The state government should release documents from the Suburban Rail Loop's control group so the public can know how much the project will cost, Mr Davis said.
"This is a basic matter of transparency," he said.
If the loop is built, trains would take three to four minutes between six stations, with a journey from Cheltenham to Box Hill totalling 22 minutes.
The eastern and northern stretches of the loop would take about 606,000 car trips off Victoria's roads per day by 2056 and the project would deliver up to $58.7 billion in benefits, according to the government.
The 90km orbital rail line's second section would run from Box Hill to Melbourne Airport.
