Construction on the light rail line between Gungahlin and the city was well advanced by then - the route was just over a year from opening - but could this new technology, which promised to be cheaper and easier, offer the ACT a new way to rapidly expand its public transport system? Could it happen without the decades-long and frequently bitter debate over light rail?

The then head of Transport Canberra, Duncan Edghill, replied to the minister's office a few hours later. There was no ringing endorsement. "Technically no doubt it could be made to work, though the dynamics would be a bit different to light rail," his email said.

"You're right to be a little wary - there's sometimes a gulf between academics and reality, and I'm sure it's more complicated than painting lines on the road. There'd still be questions of utility services, planning, stabling and depots, signalling and so forth to work through.

"But if the territory went down a [bus rapid transit] path at some point, this type of 'LRV-style' vehicle could be an option."

Much has changed since 2018. Canberra's light rail has done all that was said it would. Before the COVID-19 pandemic, its passenger numbers had exceeded expectations. The government is happy with the redevelopment and urban densification happening along the route.

Labor and the Greens were returned to government at last year's ACT election; voters picked the parties that want a light rail network for the capital. The ACT elected, again, the government with a steadfast commitment of laying steel tracks across the lake, come hell, high water or the National Capital Authority.

Meanwhile, federal Labor last week committed $6 million to plan a trackless tram route in Melbourne, connecting Caulfield Station and Monash University in Clayton, if it wins the next election. The city with a world-famous tram network could be about to put rubber on the road.

Trackless trams have come a long way since ACTION decked out a small fleet of Mitsubishi buses in the style of old trams in 1991 for a city loop service in Canberra. These days, the technology involves laser tracking and has the look and feel of catching light rail. Trackless trams are said to be stable and silent, just as light rail passengers expect.

The most regularly cited system was developed by CRRC in China, and unveiled in Hunan province in June 2017. They look like light rail vehicles on the road - long, articulated and elegant in a non-bus kind of way. The vehicles can travel at 70km/h, don't sway like buses, and can carry between 300 and 500 passengers.

Dr Yale Wong, an international transport policy expert and honorary associate at the University of Sydney, says the light rail debate is riddled with emotion. The trick with public transport is to give it right of way; if it's built for success, it will be successful, no matter the mode, he says.

"The actual traction technology - whether it is steel wheeled, or it is rubber wheeled, or it is trackless trams - it really doesn't matter at the end of the day, in terms of the inherent accessibility benefits of the vehicle or the mode. So much is actually tied to the design and implementation of the mode itself," he says.

But Dr Wong, who worked for Territory and Municipal Services in the ACT during the early days of light rail, says light rail in Canberra was going to work because it got the most attention.

"It's like sending one child to Harvard and sending the other child to the school down the street. It got everything, so obviously it would be very successful. Whereas the bus system ... it really struggled to get budgets and new funding," he says.

"People are conditioned in their beliefs based on what they know. What people know of buses is they get caught in traffic, the driver is rude and grumpy, you get on and it's packed and it's noisy and it's smelly. Whereas what they experience of light rail, with trams, has been almost romanticised, or it's been very, very positive in different places they've been to. This is a common feeling across the West, across developed countries."

While Dr Wong thinks forcing people to change at the city to get from Gungahlin to Woden, partly on light rail and partly on a trackless tram, does not make sense and could put off potential passengers, he also thinks the community needs to consider a different set of questions in the future.

"The community needs to have that choice around those trade offs. We need to start with a fixed budget and there needs to be that clear communication. Do we want 10 kilometres of light rail? Or do we want, say, 20 kilometres of bus rapid transit? Or do we want, say, 50 kilometres of a branded bus service?" he says.

Professor Peter Newman, a sustainability expert at Curtin University in Perth who is the go-to advocate for trackless tram technology, says he was unconvinced until he rode one in China. The ride quality of light rail is one of its key draw cards to getting people onto public transport and out of their cars. Trackless trams need to match it or will not shape a city any better than a bus.

Should Canberra have built a trackless tram? Should the next stage of the project be trackless? They are curly questions for Professor Newman, who worked on a 1991 study that concluded the capital needed light rail on the spine of the Y-plan.

"It's a hard one, because it's been so successful. And you know light rail is hard to get right but you did it right in Canberra. It seems to have done everything that was asked of it, and that's hard in a small town. But you know, it was very pleasing to see," he says.

Transport officials in Canberra have argued all the costs of moving utilities would be the same for light rail or a trackless system. "The main reason you move the utilities is so that you are not stopping a mass transit system every time you need to look in a manhole," Edghill told 2019 ACT budget estimates.

Professor Newman disagrees. His headline figure is trackless trams could be a tenth of the cost of light rail. "If you have a particular side of the road or the middle of the road that's set aside for a trackless tram, if at any point you just need to do something to a particular part of that road, you can just take the trackless tram around it. They're not as fixed. When I rode it, the driver was constantly wanting to show how he could just take over - just press the red button and take over," he says.

Transport Minister Chris Steel, however, remains unconvinced. He wants the gold standard of light rail on both sides of the lake, and does not want to see the southside short changed in a project designed for the next century. He told a 2020 light rail conference in Canberra that everyone in the room knew the truth: that trackless trams are in fact guided buses.

"It is unfortunate that this unproven technology is being used as a political tactic to delay investment and the benefits of proven LRT transit," Mr Steel said.

"These proponents of trackless trams claim they are more affordable than light rail, based on their ability to run on the roadway. However our experience with building light rail in Canberra suggests otherwise. Because we know that the trackslab and rail are only a small minority of the cost of light rail."

Mr Steel said the risk of adopting an unproven proprietary technology was also a considerable risk to the project, and the cost savings from trackless tram technology would be minimal or non-existent. Not to mention the trouble with having so few manufacturers to choose from.

"This would be before taking into account whether or not 'trackless trams' can deliver the transport benefits or the wider economic benefits that light rail has already proven it can in this city and cities around the world," he said.

Professor Newman says Canberra will not regret its light rail projects and think, in a decade's time, it should have taken the track-free option.

"If you can get the money and build it, I'm not in the position of rail bashing. ... But I do think that if you can't and you've really got to make a choice between either doing a maybe very short piece of light rail extension or a much more extensive [trackless] one that will cover a whole lot more of the city, then that's a serious trade off and you wouldn't want to regret not having enough light rail available because it's just too expensive," he says.

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