With all the debates about high speed rail and whether it should be provided or not in various countries around the world, it's difficult to decide the rights and wrongs of the issue. Let's consider a few.
In countries like Australia and the US, the population density is low and cities are widely spaced. High speed will be largely competing with airlines and it will be a question of time and convenience. The range of distances where high speed rail will make the difference is 200-800kms. Most trips above this will be faster by air.
Then there's the density of traffic. Given that newly-built passenger rail systems will never pay for themselves, you have to assume that government will buy the infrastructure so, if operators are to be persuaded to invest in trains and run them, there must be a business case. This is where the density comes in. There must be enough traffic potential to make the system worthwhile. This may be a difficult question for less populated countries like the US and Australia but, in the UK, the population density is such that new travel infrastructure between major population centres will soon be fully utilised.
The other big issue is the taxpayer contribution. Why should taxpayers contribute towards a project that only benefits a minority of them? Well, it does add more than just a set of railway tracks across the landscape. It will provide increase travel opportunities, it will reduce road congestion, it will minimise environmental damage, it will create jobs, it will take pressure off congested air space and it will provide competition and choice for the traveller.
There is also the argument about using the money to improve the existing railway system. I'm on the fence on this one. I'm not sure that the benefits can be balanced in favour of upgrading the existing system unless detailed studies are done of each location looking for an upgrade. I suspect that building new will give more benefits and give them more quickly.