About 25,300 Adelaide commuters use the affected railway lines on an average weekday.

The temporary closure of the Outer Harbor railway line and the connected spur to Grange – which began in late September – has been extended to January 15, 2018 – a delay of around six weeks from its original December 3 deadline.

And a southern stretch of the Gawler railway line between Adelaide and Mawson Lakes, which was shut down during the first half of October, will close again from 9pm this Saturday until December 5.

Free substitute buses will be available to commuters along those routes throughout the closure period.

The Torrens Junction Rail Project, funded by the State and Commonwealth Governments, will remove three level crossings and allow freight and passenger services to operate concurrently.

It will fix a pinch point, just north of Bonython Park, where the freight line and the Outer Harbor commuter line meet, commonly causing delays for freight trains and Park Terrace traffic, as well as limiting the length of freight trains on the line.

A rendering of the railway underpass. Image: supplied

Transport Minister Stephen Mullighan said he regretted the added disruption for railway passengers, but spruiked the benefits of the project.

“This shutdown will see construction works being performed around the clock to get the job done quickly and safely in this window where no trains will be running,” he said.

“By closing sections of track for an extended period we can deliver more work, faster and importantly in a safer environment for track workers.”

Mullighan said the project would improve travel times, reliability and safety, reduce congestion and eliminate frustrating delays for motorists at the level crossings.

“This project has already provided significant benefits for road users with the removal of the level crossing at Park Terrace, which stopped traffic for more than two hours every day on average,” he said.

“Taking away the need for freight trains to give way to passenger trains will improve freight efficiency and deliver more reliable and quicker passenger services.

“Removing three level crossings will reduce congestion for the more than 50,000 vehicles which use these crossings every day.”

The Outer Harbor and Grange lines will be closed until mid-January. Image: AdelaideMetro

The Gawler line, from Adelaide to Mawson Lakes, will be closed between November 18 and December 5. Image: AdelaideMetro

As InDaily reported earlier this year, the project raises questions about the future of the long-promised Port Adelaide tram line which would replace heavy rail to Outer Harbor.

Under the Government’s integrated transport plan, the proposed tram line wouldn’t use the rail line through the parklands, instead directing trams along the current route to the Entertainment Centre before switching to the rail corridor at Bowden.

However, InDaily understands the Torrens Junction project will be designed with diesel and electric trains – as well as light rail – in mind, allowing the future option of running trams through the parklands along the current rail corridor, and then via a new section of track along War Memorial Drive, past the Adelaide Oval, and then up King William Street.