Something like HSR shouldn't be measured in terms of a private company's projected NPV, because the social benefits will outweight the private benefits - meaning that government will need to take the initiative here.
This is about long-term nation-building vision and not simply short-sighted cost-benefit analysis. Our capital cities can't contain growing populations, and I think we can all agree that folks simply don't want to move out to satellite suburbs or city centres because of the time and poor transport infrastructure involved in the commute. Having an HSR system will go some way to resolving this.
Further, aviation is simply not sustainable, given its heavy reliance on fossil fuels. There is research in alternative biofuels, etc, but it's all progressing slowly. In the meantime, having more environmentally responsible options for transport, especially for short-haul trips, is the way forward.
Syd-Melb, for example, is such a busy air corridor - you can't argue that there's not enough demand. Ideally, a carbon pollution tax would also come into level the playing field for mass ground transport - it's another example of an externality, where social costs not being considered in private costs.
