They show the four "strategic options" developed for an eastern extension of the line that focuses "on the south-east and Illawarra" are from the CBD to Kogarah via Sydney Airport; Miranda via the airport; Malabar via Randwick; and a line branching off to both Malabar and Kogarah. Transport sources say a conversion of the existing Airport Line to carry single-deck metro trains is considered the cheapest and least disruptive option for extending the metro line. The Berejiklian government earmarked $3 billion in the state budget last year for Sydney Metro West, the cost of which Transport for NSW has estimated at up to $18 billion. The final construction cost will depend on the route chosen. A conversion of the Airport Line is considered the cheapest and least disruptive way of extending the metro rail network to Sydney's south east. Credit:Dean Sewell

Geotechnical drilling has also been carried out at Zetland as part of planning for Sydney Metro West. Loading Another leaked document shows that the transport agency has been considering four options for sites both near and on the site of Central Station in the CBD for Sydney Metro West. Two of the sites are near the eastern edge of the railway precinct and two on the western side. It also reveals that attempts to make provisions for platforms for high-speed rail at Central Station have complicated plans for both the metro line and large scale development of the precinct. "[High speed rail] provision impacts rail infrastructure and development," the documents state. "Commonwealth option proposes tracks below and above platforms 2-6, constraining development."

The possibility of separating the Airport Line from the suburban network, and running single-deck metro trains from Central to Revesby, was also canvassed in an options paper two years ago. Loading The rationale for converting the Airport Line was that it would free up capacity for an extra 12 trains an hour on the City Circle Line. Single-deck metro trains are also considered more suited to travellers lugging baggage to and from the airport. "[A metro line from] Kogarah via Sydney Airport makes the most sense because it's the cheapest option. They could roll it out in a single term of government because they could convert the existing heavy rail," a transport source said. But Transport Minister Andrew Constance said any prospect of extending Sydney Metro West to the city's east was well over a decade away.