It's a great idea (any new railway line is) but I confess I am confused by the figures.
It is estimated to cost between $61 and $108 billion. That is a very large wad of cash. After that, it gets interesting. Adelaide Now says:
A one-way trip from Melbourne to Sydney could be as cheap as $99, while the fare from Brisbane to Sydney could be as low as $75.
Brisbane to Sydney is about 900 km. Give or take 10 or 20 kms, that's about the distance from Hamburg to Zurich. You can catch a German ICE that distance and it will take you 8 hours exactly. You can catch one at 10.01am from Hamburg HBF and it will drop you off in Zurich at exactly 6pm. According to the DB website, it will cost you 152,40 EUR (that's $204.25 is today's money). So not quite as low as $75.
If the train runs, the question is where it will serve in between. You can imagine it stopping at Newcastle, Port Macquarie, Coffs Harbour and the Gold Coast. By way of comparison, the German equivalent serves large cities like Hannover, Frankfurt, Mannheim, Karlsruhe, Freiburg and Basel in between. That I think would tend to make the service a bit more viable. True it is that Sydney has a larger population than most European cities but not all of them are travelling to Brisbane on any given day.
If they do, these days they take the plane. Over that distance, the train would have to be very fast to compete. It has the benefit of picking people up and dropping them off in the centre of the city but that only really benefits people who either live there or have their hotel/conference/meeting there. It takes people time to get to the airport to catch a plane but it would equally take time to get to the centre of a city to catch the train. If you like in Blacktown, you can safely add an hour to your journey to reach central station before getting on the train for Brisbane.
Not only that, the planes travelling between Australia's capital cities are not exactly overflowing. Until recently, whenever I have travelled interstat by plane, it is on a Boeing 767 or similar that has three seats each side of a central aisle. More recently, I have been on slightly larger planes with a 2-3-2 configuration. If a railway line were built, I can see airlines making its life very difficult. We have seen price wars in the past. In fact, even without a price war I reckon in real terms it's cheaper to fly places than it has every been.
If the Commonwealth can find between $61 and $108 billion, rather than spend it on a single railway line connecting only some capital cities, I cannot helping thinking that the money would be better spent on some desparately overdue projects inside our cities. Sydney is finally getting its Epping rail link that has been talked about for decades. Its metro was canned before it was begun (a stupid and short-sighted decision). Compare that with Copenhagen - a city with a fraction of the population. Not only does it already have a metro (on which the Sydney one would have been based) in addition to its S-Tog, a circular extension is already being built.
Commonwealth money would extend our railway line north east to the new suburbs around Gawler and further south than Seaford where it is needed. It could also extend it further east to Mount Barker and beyond where, judging by today's newspaper, it is so desparately needed. We also still do not have a tram or railway line to the airport. That would be a far better use of all of that money.