Another route known as Metro Rapid promises a 20-minute journey at speeds of up to 130 kilometres per hour along a train line with 10 stations that takes in Five Dock and North Burwood.

Trains on the express route would travel at up to 160 km/h and those on the Metro Local South route at up to 100 km/h.

The fourth route, and the least likely to eventuate, would be Metro Local North, which would run via Drummoyne and under Sydney Harbour to Ryde in the north before finishing at Parramatta.

All four options in the Cabinet paper seen by Fairfax Media and the ABC run from Pitt Street in the CBD and have benefit-cost ratios well in excess of 1, meaning the benefits such as travel time savings for tens of thousands of commuters exceed the costs of constructing and operating the line.

The preferred option - Metro Local South - along which driverless, single-deck trains would run takes in the Bays Precinct at Rozelle, Lilyfield, Concord and Silverwater.

The 15-minute express option, with only five stations, performs the worst when subjected to benefit-cost analysis. Although the fastest route on offer with impressive time savings, it has only two points of interconnection with heavy rail or light rail networks, limiting its use.

Its benefit-cost ratio of around 1.7 is about the same as that presented in the business case for the WestConnex toll road.

The Metro Rapid option has a much higher benefit-cost ratio of around 2.5, meaning the benefits are more than twice the costs. Metro Local South and Metro Local North have benefit-cost ratios well in excess of 2.

Among the benefits identified for the project are public transport time savings, reduced train and station crowding, shorter waiting times, improved rail reliability and private vehicle travel-time savings.

The analysis raises questions about why the Metro West rail project wasn't considered as an alternative to WestConnex, given that most of its variants have better benefit-cost ratios.

Excluding it as an alternative when WestConnex was first considered made the $16.8 billion toll-road project appear to be the best available option.

An internal Transport for NSW memo released under the NSW Government Information Public Access Act refers to a Cabinet directive not to consider public transport alternatives to road projects.

It suggests Cabinet forbade the consideration of public transport alternatives to toll-road projects including the F6 Extension and the Western Harbour Tunnel and Beaches Link.

"It would be extraordinarily foolhardy to consider that academic transport economists, transport practitioners and a range of stakeholders will not raise these issues," it said.

"There is considerable political and reputational risks associated with not considering options."

The Metro Local North and Metro Local South routes would each cost up to $15 billion to build (in 2016 dollars) and around $2 billion to run over 30 years.

In contrast, the Metro Rapid would cost around $13 billion to build and the Metro Express less than $12 billion.

But the Metro Local North and Metro Local South options allow the government to recoup money from developers who buy air rights above 12 new underground stations. It boosts the expected benefit-cost ratios from around 1.8 to around 2.3.

The number of stations will also be crucial to determining the final price for constructing the line because a typical underground station can cost between $400 million and $500 million.

Transport and Infrastructure Minister Andrew Constance said no decisions had been made on either the funding mechanism or development around Metro West.

Well planned cities needed smart investment in both road and rail infrastructure. "Thanks to the strength of the NSW budget, we can invest in both. Our $41.5 billion investment in roads and rail infrastructure is evenly split between roads and public transport," he said.

While the Berejiklian government is still some time away from revealing the exact route for the new metro line running mostly through tunnels, it has named Parramatta, Olympic Park, the Bays Precinct and the central city as locations for stations.

Under its timetable, the new line will be built next decade and be operational in the second half of the 2020s. It will link to the $20 billion-plus metro railway under construction, the first stage of which from the city's north-west to Chatswood is due for completion in 2019.

The second stage will continue on to the central business district, Sydenham and on the existing Bankstown line, and should open in 2023.

The new Sydney Metro West will also dovetail with the planned 22-km light rail line to be built in two stages from Westmead and Parramatta to Olympic Park and Strathfield. Construction is due to begin next year at a cost of more than $3.5 billion.