The first stage of the north-south line is expected to run from the new airport to St Marys, where it would link to the existing T1 Western Line. Estimated at more than $30 billion, a north-south train line would be significantly more expensive and a longer term project than an extension of the South West Rail Link. The latter has been on the drawing board since 1994 and long considered the most likely first option for a train connection to the $5 billion-plus airport. The quickest that trains could run along an extended line from the airport to Sydney's CBD would be about 50 minutes. Transport planners doubt the rail links will be built by the time the airport is due to open in 2026, but expect their eventual construction to be funded partly by a "value-capture" model whereby levies are imposed on developments close to stations. There are about four alignments that a train line from Leppington station to the site of the airport at Badgerys Creek could take.

The preferred route is unlikely to be made public in order to avoid property speculation limiting the ability of the state to capture value and funnelling money raised into the projects. Planners and investors will be eager to hear about the estimated cost, funding and timeline for construction of the rail projects when federal Cities Minister Paul Fletcher and NSW Minister for Western Sydney Stuart Ayres release the "city deal" as early as next month. "Everybody is wanting to see whether it is a loan facility [for the projects] or a grant from the feds," a transport planner said. Attention will be on preserving corridors for the new rail lines, and firm timelines for when they would be built in order to give businesses certainty to invest.

Mr Fletcher has said the arguments in favour of a north-south rail link via Badgerys Creek are stronger than an east-west line from the new airport to Parramatta because of the former's "city-shaping potential". He has also made clear that the case for a train line to Badgerys Creek cannot be made on the basis of passenger traffic from the airport alone. In a speech on Friday, he said the "city deal" for western Sydney was based around the new airport as "a catalyst for economic growth and activity in the region". A spokeswoman for Mr Fletcher declined to comment on what rail plans would be included in the "city deal" but said a scoping study into the rail needs of western Sydney and the new airport had been handed to the state and federal governments. The "deals" for western Sydney and other cities such as Hobart and Darwin are based on an idea applied in Manchester and Aberdeen in the UK, and aimed at marshalling local, state and federal governments to work with businesses to trigger jobs and investment.