Its secretary Paul Nolan has revealed an ambitious plan to see the Metro system open from September 30, with the mixed use of manned double-decker trains and the Metro services from Rouse Hill to Chatswood.

“Sydney Trains would be able to operate on the Sydney Metro line and vice versa under this plan,” Mr Nolan told the Times.

“This is because both networks actually have the same track gauge and the same overhead wiring.”

Under this procedure, all services would have to be manned until upgrades to the Chatswood to Epping line were complete.

Mr Nolan’s plan would also result in the Chatswood to Epping tunnel remaining open and operational from September 30 — rather than the seven-month closure that will disrupt hundreds of thousands of commuters daily.

“It is all about the signalling connection in the tunnel,” he said. “They have to upgrade signalling in the tunnel to gain a better connection to the control room.

“The other issue is they are going to put barriers in place at each station.”

The group’s suggestion, which has been presented to the State Government and Opposition, would see the mixed use of double-decker manned trains and Sydney Metro services from Rouse Hill to Hornsby, via Chatswood and Central.

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“This could be an election winner opening this line early,” Mr Nolan said.

“With a range of trains and Metro services, we could see the fast-tracking of the entire Sydney network transition over to the Metro network, with transitional signalling upgrades taking place gradually.”

The proposal comes as state Greens MP Mehreen Faruqi called for the government to reinstate engineering expertise in transport departments following claims of a $500 million budget blowout on the project.

“The SkyTrain project was originally worth $340 million, it soon overran by $50 million due to a design dispute, and now the public has to pay out another half a billion dollars to compensate this government’s total and utter incompetency,” Dr Faruqi said.

“Just because they have enormous contingency in the budget for Sydney Metro doesn’t mean this isn’t a cost blowout.”

A Transport for NSW spokesman did not answer questions regarding the possibility of the Metro opening early, but said the project would be delivered within its $8.3 billion budget.

“The project is currently $500 million under budget and will open in the first half of next year, as planned,” he said. “Sydney Metro directly employs engineers with relevant expertise …”