On Tuesday, the Andrews government announced that if it regains power in November's state election, it would build a 90 kilometre orbital rail line. The $50 billion rail line – "the biggest public transport project in Australian history" – would be mostly underground, linking every major line from Cheltenham to Werribee. Loading Replay Replay video Play video Play video The announcement came three months before the election, although the project itself would take many decades – until 2050 – to complete. But engineers speaking to The Age suggested that the line, which would include five new stations in public transport-poor areas including Doncaster and at Monash Clayton, could be delivered in about half the projected time, with many suggesting 15 years was possible.

Some ventured 10 years. ‘‘Based on overseas experience, it may be possible to build a project like this within 15 years,’’ said Engineers Australia’s Victorian division president Alesha Printz. Loading But doing it more quickly would make it more expensive, with Ms Printz saying completing the project in 15 years would see a "cost premium to deliver under fast-tracked methods and contracts’’.

AECOM’s group director of civil infrastructure Mark McManamny said the government could ‘‘accelerate’’ the timeline ‘‘if the growth in our economy and our population demands that’’. ‘‘If they really wanted to push the envelope, you could potentially halve the time of the delivery. But there would be a number of challenges faced in that timeframe, particularly given the committed projects already in the pipeline.’’ ‘‘The government has to be really cognisant of the industry’s ability to respond to that level of construction in such a short term.’’ GTA transport consultant Will Fooks, who worked on Crossrail for Transport for London between 2007 and 2011, said a 15-year timeline was achievable but given the scope of the project, speeding up the planning ‘‘would not be prudent’’.

Premier Daniel Andrews and Transport Minister Jacinta Allan announcing the suburban rail line project on Tuesday morning. Credit:Joe Armao But even if the government wanted to build it faster, would they have the skills and manpower? There has been a near quarter growth over the past 12 months in engineering vacancies in Victoria. Infrastructure insiders say tunnelling expertise is scarce and firms are increasingly looking overseas to find skilled workers. The speed at which the project can be delivered also depends on the availability of clay, sand and stone, with the raw materials industries warning of unprecedented demand with multiple transport and construction projects now underway.

Loading Replay Replay video Play video Play video Former Labor planning minister Justin Madden said he was “enthusiastic about the project, but would like to see it before 2050.” “Population increases might again grow faster than we anticipated,” said Mr Madden, the Cities leader at Arup. Further questions remain about how exactly the rail proposal would be funded. The government has stated that it would be leaning on the private sector for support, in addition to the federal government.

Josh Sgro, partner at law firm White & Case, who advised Rail Projects Victoria in contractual negotiations with consortiums delivering the Metro Tunnel works, said he expected ‘‘big elements of it to be privately financed’’. ‘‘Developers could put in money [in return for] for the rights to develop above the station at some of these hubs,’’ he said. Given the government plans to build the rail line in stages, it was likely that the line would be divided into different packages, with each package having a unique contractual arrangement. The government might consider private public partnerships, where a private operator is paid for building and running the services, over a 25- to 30-year period, he said.

Deciding on a timeline always came down to ‘‘a trade-off between cost and time’’, he said. Questioned about the project’s timeline, a government spokeswoman said: ‘‘We haven’t wasted a minute to deliver the road and rail upgrades Victoria needs – and if re-elected we will get on with this project that will transform our public transport network.’’