Rereading the government press releases and trying to find gems between the lines definitely suggests it will be a Metro Tunnel job. They are careful to talk about tunnelling and “existing rail reservation” ONLY in reference to the area north of Sunshine. Dan is also at pains to talk about “integration” with the Metro Tunnel.
However, were this the case, there would be no reason to touch Sunshine at all. That is the only thing which I can’t quite stack up.
Realistically the Metro Tunnel option makes the most economic sense. It may not be great for the railfan, but it requires a minimum of headaches at Sunshine, Footscray and the city end. I genuinely cannot work out how it might be possible to add a track pair through Sunshine while leaving the Melton/WV corridor open without a bored tunnel all the way from Southern Cross or some horrendous 8-track flying junction (though of course I’d love to see the latter!)
As for the travel time, Sunshine-Melbourne Central is 20 minutes currently so 30 minutes Airport-Parkville is no problem. By all means it might need to run express Sunshine-Footscray to keep passenger loading sensible but no improvement in travel time results.
And in our wildest dreams, the MT can still offer 6tph Sunbury, 6tph Airport, 6tph WV, AND 6tph Melton comfortably. With a short turn back near South Yarra - as alluded to in the business case - 4 of those services can be dropped to leave paths for Gippsland trains in the east if needed.
You may not like it, but it’s a cheap and simple way to do what could otherwise be a VERY painful job.