The prospect of a new rail line through central Sydney – the first to be built for some four decades – is exercising the minds of developers and transport planners across the city.

And while the government has released maps showing possible station locations, more changes to the alignment of the project are certain before construction starts on the city's new "metro" line in 2017.

Transport Minister Andrew Constance and project director Rodd Staples this week started consulting the transport industry over the construction of the "metro" line, which is to run from Chatswood in the city's north to Sydenham in the south where it will connect to the Bankstown Line.

And both north and south of the harbour, the project's basic planning and geography indicate numerous extensions are likely to be added to the line.

For instance, Transport for NSW says as many as 30 trains an hour – one every two minutes – will be capable of running on the new line through central Sydney.

That is obviously more trains than will be needed for the line's only current extension north of the harbour – the northwest link to Kellyville and Rouse Hill.

The most likely second northern extension, therefore, would be somewhere under Military Road, through Neutral Bay and Cremorne and perhaps to the Spit.

This route has been included in past planning documents, and at this week's industry briefing the government stressed it was "building the foundation for more metro lines in the future".

South of the harbour, Transport for NSW has said it faces a choice between a station at Sydney University or Waterloo, where the government developer UrbanGrowth NSW is pushing for a station to service new apartment blocks.

The university's director of campus infrastructure services, Greg Robinson, said the "most likely location of us is actually near the Seymour Centre, near City Road."

Mr Robinson said the university could contribute land to the project, and the station could serve as an interchange for City Road buses. The university already plans to redevelop the area around the existing Faculty of Architecture building.

But if the government chose this option – and a decision is due before year's end – this would leave a gap of almost five kilometres before Sydenham Station, the next stop on the planned route.

Urban planning consultant Alex Gooding said that would be an extremely large gap for a metro system, which typically have stops close together in dense areas.

If the government were to fill that gap with another station, it could build a stop at the existing Newtown Station, providing a connection from the metro route to the Inner West line, or in Enmore or north Marrickville to provide rail services to new areas.

Both Mr Gooding and transport expert Garry Glazebrook said the government should build a second metro line south of the harbour, rather than solely connecting to existing heavy rail lines.