Transport bureaucrats have warned that passenger capacity on parts of western Sydney’s rail network will be exhausted by the late 2020s, and work to upgrade existing train lines in the region needs to start without delay.

A second batch of leaked portions of a cabinet-in-confidence document first reported by the Herald last week also reveals Transport for NSW is planning to extend the T5 heavy rail line from Leppington to Bradfield, near the new airport, over the next six to seven years, ahead of a potential extension from Parramatta to Epping by the end of the next decade.

The revelations come as Premier Chris Minns lays the groundwork for the public release of an independent review into the state of the city’s existing double-deck train network by warning on Monday that it makes for “sobering reading”. In doing so, he has dampened hopes his government will embark on another spending spree on new driverless metro lines.

In warning of the need for upgrades, Transport for NSW has proposed spending between $7.2 billion and $10 billion over the next 15 years on improvements to the T1 Western and Richmond line.

“The population and employment growth currently occurring within Greater Parramatta and the Western Parklands City highlights the importance of the efficient movement of both people and goods within the area,” the department’s confidential medium-term rail plan states. “Existing capacity is expected to be exhausted by the late 2020s, resulting in uneven loads and unreliable sources.”

The department stressed the need for planning to start “imminently” so its timelines for rolling out the upgrades could be achieved.

While two new metro rail lines will help, the department warns major improvements to the heavy rail network will be needed to transport commuters between the city’s outer west and Parramatta.

“Adjacent rail services will be required to properly integrate the metro corridors into the existing rail network,” it states.

As part of a staged approach, it has proposed major upgrades at Central Station to boost capacity on the T1 line to 40 trains an hour, followed by “targeted capacity and reliability improvements” along the Richmond rail corridor. The latter will result in two extra direct services to the CBD from Vineyard and Riverstone during peak and off-peak periods.

A chart of proposed investments shows the Transport department plans to spend the bulk of money on a “new Cumberland line program” between 2029 and 2033. That would involve extending the line used by double-deck trains from Leppington to a site just south of the new city of Bradfield.

A confidential review of Sydney Metro two years ago proposed completing an extension of the airport metro line from Bradfield to “Bradfield South” by 2032 at a cost of $2.3 billion, and the heavy rail line from Leppington the following year for $4.6 billion.

It would connect the airport metro line to southern parts of the city’s double-deck train network, offering an alternative way for passengers to access Western Sydney Airport by rail. At present, the only connection will be via St Marys, which is to the north of the new international airport.

In its latest plans, the department has also floated the idea of a new rail line between Westmead-Parramatta and Kogarah, which it has slated for about the early 2040s.

Transport for NSW said in a statement that it was its role to develop long-term plans for a growing city like Sydney to ensure the public transport system grew.

“This draft plan has not been approved by the NSW government and none of the new lines on this map have been costed yet,” it said.

Business cases for potential extensions of the Western Sydney Airport metro line northwards from St Marys to Tallawong, and south from Bradfield to Macarthur, are due to be completed by early next year.

The idea of building heavy rail from Parramatta to Epping was proposed in the Carr era and shelved by the same government in 2003.

The Sydney Morning Herald has opened a bureau in the heart of Parramatta. Email parramatta@smh.com.au with news tips.