He has flagged a cities policy, to be released before next month's budget, that is intended to transform the way Australian infrastructure is funded. He will propose the use of high-speed rail developments to encourage population growth in regional centres and ease pressures in Melbourne and Sydney.

Mr Turnbull clearly sees himself as a nation builder. Yet it will be interesting to see whether he is willing to commit funds to match his rhetoric. He should expect criticism from so-called free enterprise think tanks about such use of public funds. On high-speed rail, they will have a case to argue: every time it's been raised, the overall costs have worked against it.

Nonetheless, Canberra, Goulburn and Yass would greatly benefit from a high-speed rail network along the eastern seaboard. But would any federal government risk the ire of Canberra-hating shock jocks – not to mention large sections of the Nationals – by building the Sydney-Canberra section first?

The long-running national debate over the very fast train is analogous to the long-running local debate over Canberra's transport infrastructure. Indeed, Mr Turnbull could look to the national capital for inspiration on rail. The ACT government is building the first stage of its light-rail network, and without federal funding. Putting aside the question of whether this is the best use of ratepayers' money for transport, former chief minister Katy Gallagher deserves some credit for making a tough decision – one previous governments and oppositions baulked at making – which she knew would cause her electoral grief.

While critics argue Labor's hand was forced by the Greens, the reality is the Labor administration has kept its nerve, against strident criticism (including a majority of letters on the issue sent to this newspaper). Ms Gallagher's decision was preceded by decades of argument, and indecision, over the best way to implement a rapid, mass-transit system in a city with far-flung suburbs and modest population.

In the national debate, the Hawke government decided against the huge subsidy needed from taxpayers to build a very fast train. And little has changed, despite the idea being floated before elections by Kevin Rudd, John Howard and now Mr Turnbull.

But who, apart from desperate politicians, is clamouring for this nation-building project? Could it be at least partly funded by "value capture" of developments along the corridor? Whereas Europe has relatively short distances and large population centres, Australia does not. Others argue that our manifest embrace of cars weighs against any economic benefits of a very fast train.

Whatever Mr Turnbull decides, he should not allow his decision on high-speed rail to be influenced by the at-times obsessive interest in public debt that has been dogged by Coalition in recent years. Money spent on useful infrastructure is money invested, not wasted. The unanswered question is just how useful fast rail will be.