Even if the train follow the existing highway it would do pretty well - it's almost entirely straight and flat.
Sydney is getting a 2nd airport. Air travel is easier to build, easier to expand and requires no private property resumption, wildlife corridors, bridges or tunnels.
There is also zero population density between the two cities.
It won't happen.
Even if the train follow the existing highway it would do pretty well - it's almost entirely straight and flat.