Let's have a reality check here. The only reason the government is proposing this, is to avoid the most bleedingly obvious solution of extending the SWRL to the airport, which just doesn't fit in with their metro expansion agenda. A rail link to St Marys may well be warranted as part of long term rail links on the outer fringe of Sydney, but it's not metro territory.
The more immediate focus should be on connecting the new airport with the CBD via the existing SWRL and East Hills Line express tracks via both Sydney Airport and Sydenham. It can also provide an interim rail connection between the airport and major Western Sydney centres such as Campbelltown, Liverpool and Parramatta. It doesn't preclude compatible SD trains being introduced to suit air travellers with luggage at both airports. The rest can be served by bus until patronage warrants a further rail connection.
It's rather simplistic to suggest that the new airport will only be catering for Western Sydney travellers. With the likelihood that it could become a hub for budget airlines, it will attract travellers from all over Sydney as well as Regional NSW. Add to that in-bound tourists, whom I suggest would mostly be destined for inner Sydney, as well as its advantage of being a 24 hr airport.
A business case is currently being undertaken by a Joint Federal/State committee into the Badgerys Creek to St Marys Metro link, but I am fearful that it will not be an objective analysis, including the option of extending the SWRL to the airport, which should be the logical outcome. It is more likely that they will just rubber stamp what the NSW government wants. The Feds wouldn't have a clue. I also expect that Infrastructure Australia will rubber stamp that decision, because they are not the independent agency they are purported to be, but tow the Federal government's line.
Badgery’s Creek airport rail link neglects CBD