[11] In 2013, a proposal was raised to extend the then-proposed North West Rail Link, by building a metro-style tunnel from just south of Chatswood Station via St Leonards and North Sydney and under the Sydney Harbour towards Central and Redfern, before joining the newly converted metro lines towards Hurstville, Bankstown, Lidcombe and Liverpool.

[16] In June 2017, a John Holland, CPB Contractors and Ghella joint venture was awarded the contract to build the twin tunnels from Chatswood to Sydenham.

In November 2018, a $1.376 billion contract was awarded to an unincorporated joint venture between CPB Contractors and UGL to be known as Systems Connect, which will include the laying of track from Chatswood to Sydenham as well as converting power on the Bankstown line to Metro standards and also the building of further Metro train facilities at Rouse Hill and Marrickville.

The conversion and incorporation of this section of track into Sydney Metro has not been without controversy, with a letter written by four former rail executives John Brew, Ron Christie, Bob O'Loughlin and Dick Day casting doubts on the government's claims of improved reliability as well as warning that commuters west of Bankstown face additional interchange for travel towards the City Circle.

[24][25] The NSW Legislative Council announced on 22 August 2019 an Inquiry into the "Sydenham - Bankstown Line conversion"[26] examining decision making in relation to the transport project.

The Inquiry to be conducted by Portfolio Committee 6 - Transport and Customer Service and chaired by Abigail Boyd released its report on 9 April 2020, recommending that the conversion not take place with Sydney Metro instead to terminate at Sydenham.

[30] In April 2023, it was confirmed by the government that the conversion of the Bankstown line was delayed by at least twelve months and was expected to open in 2025, after the main CBD tunnel commences operations.

Potential additional stations were also proposed for the industrial area of Artarmon (underground), St Leonards, Barangaroo and either the University of Sydney or Waterloo.

[41] The study's final report was released in March 2018 and included a proposal to extend the Sydney Metro City & Southwest from Bankstown to Liverpool.

It was to have provided the centre section of a planned North West-CBD-South West rail arc connecting major areas of employment with the CBD and airport.

The proposal was announced by Labor Premier Bob Carr on 15 June 2005 (shortly before his resignation), and formed part of the $8 billion Metropolitan Rail Expansion Project (MREP).

The six kilometre CBD Rail Link was slated to cost $5 billion, and was to include duplicated tracks on the North Shore line between St Leonards and Chatswood.

It was to provide a second railway line transversing Sydney Harbour to ease congestion at Town Hall and Wynyard stations, both considerably crowded and unable to be easily expanded, and to reduce travel times between the city and the lower North Shore.

A previous MetroPitt proposal, travelling further east (and under its namesake Pitt Street) and through the financial district, could also be built deeper underground, limiting the impact on existing infrastructure.

In 2010, the new Keneally government cancelled the entire metro project, and the CBD Relief Line & Western Express concept was announced as replacement.

The line would have offered an alternative heavy rail link in the CBD between Redfern and Wynyard, but also did not cross the harbour into the North Shore.