The conversion of the Bankstown line, along with the Metro West project between the CBD and Westmead, had been under threat of cancellation after mammoth cost overruns prompted NSW Transport Minister Jo Haylen to launch a review.
But the final stage of the City and Southwest Metro project has been spared the axe, after Ms Haylen and Premier Chris Minns released an interim version of the review on Tuesday.
Speaking in front of a gleaming new metro train, the pair announced an extra $1.1 billion in funding - diverted from other areas of the transport budget - to ensure the 13.5km line opens between July and October 2025.
The process won't be without difficulty, as commuters must endure 12 months of replacement buses while the track is closed for construction.
Alternative plans, including a 15-month partial shutdown and even longer disruptions, were deemed unfeasible.
Ms Haylen did not sugarcoat the disruption southwest Sydney residents will face as a result.
"This conversion won't be easy," she told reporters.
Approximately 100 extra buses will run replacement routes, including express services, but long delays to travel times are inevitable.
It won't be the first time commuters are forced to suffer a rail closure while an existing line is converted to metro.
Buses replaced trains for eight months when the Epping to Chatswood rail link was upgraded in 2018 as part of Metro Northwest.
When opened, the $21b City and Southwest project will extend the Chatswood to Tallawong line to create a continuous 66km ribbon of railway.
Business Western Sydney director David Borger welcomed the announcement, saying billions of dollars have already been spent.
"Let's finish the job and make sure we use this big investment to drive new homes, jobs and amenity for the communities along the upgraded and improved rail line."
Current 35 minute travel times between Bankstown and Central will be slashed, with single-deck trains running every four minutes during peaks.
Shifting traffic from the existing heavy rail line onto the standalone metro promises to clear a bottleneck in the congested city circle and significantly improve reliability across the network.
Several recent incidents have ground Sydney's tangled rail system to a halt, leaving commuters stranded and damaging confidence in ageing infrastructure.
"This is great news not only for the communities of southwest Sydney but for all passengers across our train network," Ms Haylen said.
Testing of trains on the Chatswood to Sydenham leg has already begun, with the line set to carry its first passengers between July and October 2014.
Questions persist around the $25b Metro West, with a decision into the under-construction line's future not due until after the September 19 budget.
The review flagged moving funds from the project to the Bankstown conversion "is highly likely to impact to the target 2030 opening date" and further increase costs.
Mr Minns did not rule out introducing value capture - in which businesses that will benefit from the line contribute to its cost.
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