As emergency exits are either end of the train it must have at least one central aisle big enough for a wheelchair.
You would be right in alternately skipping Albury and Canberra to get under that magical one hour but I imagine many people would be only going Sydney or Melbourne to Canberra and having a regular frequency would be somehow better for them.
I am thinking about starting a Magtube site on the internet with a Magtube train on Google 3d and this may have luggage racks above the centre aisle as there is no need for overhead wiring and also more space for bikes, electric scooters and toilets even if it needs an extra carriage to maintain capacity.
Regarding a platform either side we must remember that stations would probably be underground and that segmental linings would continue through them with only inner doors on the carriages lining up with outer doors on the segmental lining when the train is in any station with a 10m wide platform between them.
This could be done without any pillars to support the roof but any wider you would have to use them and as the train would be sitting there 15 minutes there would be ample time for people to exit or board.
Unless people here and on other boards can agree on the best way forward I see nothing ever being done either to Magtube or the existing Sydney/Melbourne track when really both need doing.
A rather radical thought to do both at the same time over a ten year period is for us to build some sort of long bridge for the existing rail so the subgrade could be stabilised and track lowered to allow clearance below bridges at the same time as trains could slowly pass over it.