With the NSW government investigating faster rail to Canberra, and with federal Labor proposing intercapital high speed rail as part of the COVID-19 recovery process, can we be confident a rail upgrade, on this and other routes, will not be postponed again?

A starting point is recognising that, in Australia's geography, faster, or fast rail, with maximum speeds between 200 and 250km/h, and high speed rail, with top speeds around 350km/h, are not substitutes.

With different operating speeds, each affects "time-distances" between places differently. So impacts on policy outcomes - on decentralisation of housing and employment, which can contribute to easing pressures on capital city infrastructure and liveability and on intercapital passenger capacity and connectivity - also differ.

With fast rail, locations up to about 100 kilometres from a capital city - such as Gold Coast (Brisbane), Wollongong (Sydney) and Geelong (Melbourne) - would fall within a one hour service time. They could become both more 'commutable', with resulting new housing development, and more suitable for relocation of office-based and other employment.

Locations between 100 and approximately 300 kilometres from major capitals, including Canberra and Albury-Wodonga, could receive two hour services. While too far from Sydney and Melbourne respectively for large-scale commuting growth, these cities could experience business and employment growth, including along connecting corridors, as we suggested in The case for fast rail to Sydney, (The Canberra Times, May 7).

International precedents matter and evidence from the United Kingdom, not cited by the Grattan Institute in its recent Fast train fever report, which queried the benefits of fast rail upgrades, is encouraging. In Australia, it is primarily larger centres such as Newcastle, Wollongong, Geelong and Canberra, all within 300 kilometres of a major capital, that would have the infrastructure base and labour market strength to attract and retain households - preconditions of sustained employment growth.

So fast rail should be 'fast enough' to achieve population and employment decentralisation objectives, with support from complementary planning-related policies.

High speed rail, in contrast, could provide service times of less than three hours between Melbourne and Sydney, and Sydney and Brisbane, with route distances exceeding 800 kilometres, according to a 2013 study commissioned by the federal government.

High speed rail's journey time advantage presents an opportunity to supplement air transport capacity into central Sydney, Australia's leading commercial centre and the destination for more than half of domestic visitors to the Sydney region. Sydney region airport capacity will increase when Western Sydney International Airport opens in 2026. However, it is unclear how attractive its air services may prove for many domestic business visitors, for whom it could add an hour to a round trip. So the opportunity remains for high speed rail to provide additional 'air transport-like' capacity, with convenient city centre accessibility.

High speed rail services might, separately, target commuting from beyond a 100 kilometre capital city radius. Locations and travel markets would likely be small, at least initially. Investment costs could, in principle, be funded by value capture revenue from land development for housing within the catchment of stations.

Using this model, the private Consolidated Land and Rail Australia consortium plans eight new cities, linked by high speed rail. In the intercapital market, where at least partial public funding would be required, the capital cost of high speed rail, at several times that of fast rail, may prompt delay, until implementation becomes unavoidable to preserve efficient passenger connectivity.

However, fast rail, involving upgraded tracks and faster trains, will still be needed if and when high speed rail is constructed, operating on new, dedicated infrastructure. Whereas intercapital high speed service times would constrain the number of intermediate stops, fast rail would permit more stops in more places. This aids rebalancing of population and employment growth patterns. Freight trains would also benefit from faster transit times, on tracks they would continue to share with passenger services.

Fast rail and high speed rail need no longer be 'either-or' in planning Australia's future transport infrastructure.