The corridor is in scope for the NSW government's fast rail strategy and is also listed for a later phase of the federal government's faster rail plan. This involves improved passenger rail services to regional centres in the orbits of Sydney, Melbourne and Brisbane, to help reduce pressures of congestion and housing unaffordability in those cities.

The Sydney-Canberra route is longer than others identified by the federal government, such as Brisbane-Gold Coast, Sydney-Newcastle and Melbourne-Geelong, and is also, uniquely, a "top 20" domestic air route. Particularly given current circumstances surrounding Australia's aviation sector, clarity about the economic and strategic case for following through with the Sydney-Canberra upgrade is important.

Here are four reasons we should back the project.

Firstly, bringing the Southern Highlands within around a one-hour commute time to Sydney would encourage new residential developments, offering more affordable homes with access to Sydney jobs. This would help relieve city pressures, while also growing a larger population base for locally based industry, similar to the strategy for other proposed upgrade corridors.

Secondly, a two-hour rail service between Sydney and Canberra, Australia's eighth-largest metropolitan area, could create new opportunities for "knowledge-intensive" businesses in both cities, helping them to organise their activities differently, achieve scale economies and become more productive.

A two-hour rail service would compare in overall journey time with air transport and would allow businesses to capture benefits of proximity that are out of reach due to air transport's high cost.

To illustrate, full-service airfares between Sydney and Melbourne are pitched similarly to those between Sydney and Canberra, despite the former route being three times longer than the latter. Moreover, low-cost airline (Jetstar) services between Sydney and Melbourne are priced at around a third of Sydney to Canberra full-service fares.

While low-cost carrier entry would be welcome, a key source of the cost differential is the "up then down" character of the Sydney-Canberra route, where, as with all air routes, costs such as check-in, baggage handling, take-off and landing are intrinsically airport-linked.

In contrast, fast rail, offering a maximum speed of up to 250km/h, would be a more natural fit for a high-capacity 300-kilometre route. Fares might be set a little above low-cost airline levels, as on the United Kingdom's rail network, or below this level, as in Sweden. Fast rail would not replace air transport, as high-speed rail, with around a one-hour service time and at much higher cost, might largely do. But it could nevertheless grow business and tourism markets. The improved and straightened track that fast rail requires might also provide opportunities for rail freight services.

Thirdly, a Sydney-Canberra fast rail service would help alleviate planning, housing and congestion pressures in Canberra, by making the Goulburn region commutable to Canberra on a larger basis. This would serve to underpin Canberra's future labour market and economic growth.

But is Canberra large enough for the security of its growth path to be considered nationally important, similarly to those of Sydney, Melbourne and Brisbane? Taking into account Canberra's location and role within a broader region, the answer is yes.

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As a large inland city, smaller population centres surround Canberra in all directions and rely on it for access to goods and services. Bus routes to and from Goulburn, Batemans Bay, Yass and Wagga Wagga are just one indicator of this regional role. Assuming there is demonstrated mutual benefit for both Goulburn and Canberra from a Sydney-Canberra fast rail service, a similar Melbourne-Canberra service, through a new link, say, from Gunning to the city's north, could follow.

As Gabrielle Chan writes in her 2018 book Rusted Off: Why country Australia is fed up, "the possibilities of opening up the town would be endless, if ... there was a train from Canberra to Harden Murrumburrah, a 90-minute drive. It would be the equivalent of commuting from the Blue Mountains to Central Station in Sydney. It would provide cheap housing for young families in Canberra."

A final reason to persist with a fast rail upgrade of the Sydney-Canberra rail link is the opportunity to progress towards a low-emission service, so reducing the nation's carbon footprint.

With over 100 passenger and freight train movements per day, the section of track between Macarthur, at the end point of the Sydney urban network, and Goulburn is the most heavily used of the non-urban sections of the interstate rail network that connects Brisbane, Sydney, Melbourne, Adelaide and Perth. The section's usage would increase with a more frequent passenger service between Sydney and Canberra.

Whether or when usage would be dense enough to warrant extension of the electrified network from Macarthur to Goulburn would be for evaluation. Together with eventual transition to decarbonised electricity generation, electrification would enable zero-emission powering of trains on two thirds of the route, as also for Sydney-Melbourne trains and other trains using this section of track.

Fast rail between Sydney and Canberra will call for a staged approach over time, but has a range of national, regional and network benefits.