Trains would run up to 130km/h through dedicated tunnels under Labor's $50 billion suburban rail vision for Melbourne, branded by the opposition as unrealistic.
Premier Daniel Andrews released the travel times on Wednesday, saying trains would run with no level crossings or interaction with the existing network.
Work would start in 2022 with the project to be completed by 2051.
However, the opposition says the unfunded pre-election announcement is unrealistic.
"I don't knock new infrastructure, I don't knock the idea," Opposition Leader Matthew Guy said on Wednesday.
"But I do question the seriousness of the announcement when it will cost $50 to $90 billion. That speaks for itself about how realistic it is."
The 90km underground train route would link Cheltenham in the southeast to Werribee in the southwest and connect to the proposed Melbourne Airport rail line, with up to 12 new underground stations.
The trip from Cheltenham to Melbourne airport would take 45 minutes according to modelling, Mr Andrews said.
"Some projects are just so compelling ... our politics need more projects that the person who starts them won't be there to cut the ribbon," he said.
"That's what our politics need, that's what our city needs."
The trip from Box Hill to the airport, and the trip between Monash University in Clayton and La Trobe University's Bundoora campus would both be about 25 minutes, he said.
The state government vowed $300 million for a business case if re-elected in November.
But station locations, route alignment are still to be confirmed and significant commonwealth investment would be required.
"It is really important not necessarily to give away exactly where the new stations will be as I don't want property developers ... buying up land," Mr Andrews said.
He said specific details had not yet been given for competitive reasons.
The state government says Monash, Deakin and La Trobe universities would be connected via the route in addition to TAFES such as Box Hill Institute, Melbourne and Victoria University polytechnics.
Regional passengers would also be able to go to superhubs at Clayton, Broadmeadows and Sunshine rather than having to first go to Southern Cross Station.
"This project will change the way people think about their lives and their future - opening up jobs, study and opportunities that simply aren't an option now because it's too hard to get across town," Mr Andrews said.
