SRT Central station: Airport line becomes branch of SRT?

This is now my third post on speculation about the design of SRT CBD stations, with my previous posts looking at Pitt St and Martin Place stations.  This post will look at the SRT Central station.

My best guess for how the SRT Central Station would work comes from the NSW 2007 Infrastructure SEPP, which reserved a corridor for the CBD Metro.  Although the CBD Metro ended up being cancelled, it's core idea of a segregated single deck system was subsequently modified and adapted and survives (controversially, but also cautiously given a tick of approval by others) in today's NWRL and the planned future stages of SRT.

So what does the old CBD Metro corridor say about a possible SRT station?  Firstly, it is likely to be built on the adjacent west side of Sydney terminal, making interchange with inter-city & country trains from Illawarra/Blue Mountains/Central coast quite efficient:

Interchange with buses at Railway Square should also be quite good.  Others have speculated that the station will be built under the country trains platforms, but this would be a more complex and disruptive possiblity, but it would make country/intercity train interchange even better.

One problem with this location is that it's very hard for interchange with airport line trains, which TfNSW has "secretly" been planning to terminate at Central as a shuttle service.  These secret plans entail building a new platform for terminating Airport line trains, so where/how could this new platform be constructed?

A possible clue could be come (hidden in plain sight!) from the official PR material released by the Baird government on it's Rebuilding NSW website - specifically, page 10 of the Sydney Rapid Transit brochure mentions:

"Initial engineering studies indicate the optimum way to build SRT will be with four tunnel boring machines"

Why is this significant?  It indicates that there has to be a TBM (tunnel boring machine) launch site somewhere near the middle of the route.  Prince Alfred Park, located right next to Central Station (and importantly right next to the Airport line tunnel portal) is one logical possibility.  As shown in the map below, launching a pair of TBMs at Prince Alfred Park may allow a short extension of the Airport Line to run under Central station before continuing on as the SRT tunnel into the CBD and then North Sydney.  The other pair of TBMs would launch presumably from Sydenham, and come up to Central as shown:

This all depends on more detailed engineering studies confirming that depths/gradients line up, as metros can only cope with a maximum gradient of 4.5%.  It also raises the intriguing possibility of the Airport line actually becoming a branch line of the SRT, rather than terminating at Central.  This might explain why the Hurstville branch of the SRT has quietly been dropped off TfNSW PR documents on the SRT.  One can take speculation only so far, but the Airport line has always made more sense as a single deck metro operation, running through the southern arm of Sydney's global economic corridor and connecting major centres like Green Square and of course the airport international and domestic terminals.

If this is all so logical, why haven't TfNSW canvassed this option in public?  Maybe it requires more detailed engineering studes.  Or maybe it is due to requirements of the existing Airport Line PPP (Public-Private partnership) not allowing TfNSW to make any official decisions about the Airport Line, and to take up any public commitment would weaken TfNSW's commercial negotiating position.  With the PPP due to expire in 2030, TfNSW has a bit more time to wait before it can consider restructuring of the Airport Line.