Work needed for an upgrade to the Wollongong to Sydney rail link that would deliver passengers to Central in 66 minutes produces benefits at up to twice the cost according to a previously unreleased Cabinet in Confidence report.

The proposed schedule, also seen by Fairfax Media, has express passenger trains taking 20 minutes to get from Wollongong to Helensburgh, 35 minutes to get from from Wollongong to Hurstville, 54 minutes to Wolli Creek, and 66 minutes to Central.

Wollongong to Sydney in 66 minutes possible with a Maldon to Dombarton freight link. Credit:Rob Homer

The main expenses would be a $2.9 billion tunnel from Thirroul to near Waterfall on Sydney's southern tip and completion of the long-stalled freight link from Dombarton near Dapto to the commuter line.

Costed at between $700 million and $800 million in the Cabinet in Confidence document seen by Fairfax Media, the freight link has a benefit-cost ratio of between 1.5 and 2.4 depending on the scenario chosen.