Australia needs to get its act together and build high speed rail lines. Their geography is actually fantastic for it, its criminal not to have done it 40 years ago.
Why? What problems will this solve? Much better to spend it on public transport improvements in our cities, and a congestion charge to make cars pay for what they use.
And the main thing holding back cities is cars and removing them is not primarily a cost issue, but a political one.
As for what will it solve, it decarbonize the most important air routes in your country. It will serve as a backbone for the transportation network of your country for next 100 years.