The demise of the plan to run single-deck, driverless trains on the existing Illawarra Line to Hurstville means the government will need to re-think where it will run a second metro line south of Sydney Harbour.

When former transport minister Gladys Berejiklian announced "Sydney's Rail Future" plan for the train network in 2012, she said a second harbour rail crossing would extend the metro system in the south to both Bankstown and Hurstville on existing lines.

In 2014, the government said it would fast-track the metro extension to Bankstown and, on the latest timing, that project should be completed by 2023.

But the government has never formally dropped the plan to run the new metro trains on the existing Illawarra line to Hurstville.

The decision, wherever it falls, is crucial.

Because the new metro line is being built with the capacity to run trains every two minutes, the government needs to nominate a second destination for those trains south of Sydney Harbour, as there is plainly no need to run trains to Bankstown every two minutes.

To the north, the metro trains are to run to Rouse Hill in the city's north-west. But there are longer-term plans to build a second branch of the metro line under Mosman in the direction of the northern beaches.

The metro system will operate single-deck trains with significantly fewer seats than Sydney Trains' fleet of double deckers which ply the city's existing rail network.

Transport for NSW insists that an extension of the metro line to Hurstville is still part of its long-term thinking.

"Hurstville remains a long term option for Metro," a Transport for NSW spokeswoman said. "Our priority at this stage is accelerating delivery of metro in the north west, through the CBD and Bankstown."

Yet transport sources and government planning documents show a list of problems with the proposed scheme.

An application from Sydney Metro says that converting the Illawarra line would "require additional tunnels and tracks, significant enabling works such as alternative freight routes and a new train stabling and maintenance facility" for Sydney Trains, which operates separately from Sydney Metro.

It also emphasises that converting the Bankstown Line would be "less complex to convert and segregate" from the existing rail network than other lines.

Transport insiders say long-term obligations for freight trains to run on the existing lines between Sydenham and Hurstville "rule out metro as an option" on the heavily-used rail corridor.

A spokesman for Transport Minister Andrew Constance said any future decision on possible extensions to Sydney metro "will be made when the proper planning process has been done".

"We're committed to stage one of the project in the north west ... and we're also cracking on with stage two, delivering metro rail through the CBD and out to Bankstown," he said.

While the government has fast-tracked converting the line from Sydenham to Bankstown to metro services, plans to extend it to Hurstville barely rate a mention nowadays.

Senior transport bureaucrats have been less forthright in their views on the extension south to Hurstville in recent times.

"In terms of Sydney's rail future, that is in there at this point in time," NSW Transport Secretary Tim Reardon told Fairfax Media in August. "We will see where we get to over the next few months."

The government has been under pressure from councils in Sydney's south to extend the metro line to Hurstville.

The first stage of the $8.3 billion metro line in Sydney's north west from Cudgegong Road to Epping and Chatswood is due to open in early 2019.

The cost of the second stage of the project from Chatswood to Bankstown via Sydney's CBD has been put at up to $11 billion.