The province is proposing to build the Downtown Relief Line using unspecified “alternative delivery methods” as part of its proposal to take ownership of Toronto’s subway.

The vague and unexpected statement was among major changes to four projects the province outlined in a letter to the city released Tuesday. The province went on to say the relief line would be a “truly unique transit artery” and “free-standing project” separate from other parts of the “technologically outdated” subway system.

The relief line, which the city has said could open as early as 2029, is planned to run across Queen St. downtown, north to Pape station, and, in Phase 2, further north to Don Mills station to take pressure off the bursting Yonge line. Previous estimates put the cost at $6.8 billion. The province says costs have doubled. The city disputes this.

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But what does this all mean? And what kind of project could they build beside a traditional subway?

We took a look at public transit projects around the world and came up with some options:

Boston, Mass.: underground bus rapid transit

Boston’s “Silver Line,” constructed in phases beginning in the early 2000s, incorporates buses that duck beneath the road’s surface.

Silver Line buses operate partially in underground tunnels, said Massachusetts Bay Transportation Authority deputy press secretary Lisa Battiston in an email.

Using buses both below and above ground was an “economic and geographically strategic decision” for the city, she said. The line’s tunnels operate beneath the Boston Waterfront area and let buses avoid street-level bridges, she added.

The vehicles, which take passengers all the way to Logan Airport from South Station in the city centre, also make connections with the traditional subway system. When they’re above ground they sometimes have their own lane, making it easier to escape traffic.

But Ryerson City Building Institute executive director Cherise Burda says tunnels are the most expensive part of subways. So using buses instead of subway cars on the Relief Line wouldn’t save much money.

Vancouver, B.C: SkyTrain

Most of the transit system in the west-coast city is elevated, but some of it does run underground. It’s also completely automated and driverless, says spokesperson Lida Paslar.

It connects downtown Vancouver with Burnaby, New Westminster, Surrey, Richmond and the Vancouver International Airport. The first line was built in 1986.

Matti Siemiatycki, interim director of the University of Toronto’s School of Cities, said the elevated portions of the SkyTrain are the “followup technology” to Toronto’s Scarborough RT.

“It’s a number of generations down the path from that,” he said.

Seattle, Wash.: Center Monorail

According its official website, Seattle’s monorail was built for the 1962 World’s Fair.

It now provides a quick, car-free connection between downtown Seattle and Seattle centre, an area home to tourist attractions such as the Space Needle.

The monorail has become a beloved landmark, and carries more than 2 million passengers a year, fitting up to 250 per train. But it only goes about 1.4 km.

Monorails, sometimes found in amusement parks, also run in cities from Mumbai to São Paulo. They run on a single rail and typically go more slowly than elevated rail like the SkyTrain.

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Premier Doug Ford has shown an affinity for monorails in the past — one along the waterfront was part of his vision for the area as a city councillor in 2011.

But Burda said, given the proposed route of the relief line, any form of elevated rail or monorail would require heavy construction and intervention in densely populated, older neighbourhoods such as Leslieville.

“If you’re going to stick an elevated system above those streets you’re going to have a lot of opposition,” she said.

Zurich, Switzerland: Light rail transit with dedicated lanes

Streetcars in lanes without cars are already part of Toronto’s transit network.

But they’re much more common in European cities such as Zurich, Switzerland, where they run like clockwork and also rarely break down or short turn.

This type of transit works best on wider avenues, says Burda, and given Ford’s history of prioritizing cars on the road, this is highly unlikely as an option to replace the Downtown Relief Line.

Montreal, Que.: Le Réseau express métropolitain (REM) public private partnership

This 26-station, 67-km light rail network will link downtown Montreal with suburban communities of the South Shore, West Island, North Shore and the airport. It promises to be electric and completely automated, and will run on the ground, underground and overhead.

The estimated cost is about $6.3 billion, with $2.95 billion of that covered by the Caisse de dépôt et placement du Québec, the province’s leading pension fund company. The governments of Canada and Quebec will chip in another $1.28 billion each, according to La Caisse’s website.

This kind of funding model and independence from the rest of the transit system might be what the province is getting at with its phrasing in the letter, says Siemiatycki.

Ryerson’s Burda agrees the wording could hint at interest in a public-private partnership, or just be punting the project down the line.

“I would hazard to guess that that’s just a placeholder for, let’s find different ways to fund this and build this more cheaply, so that we can prioritize the things we really want to build,” she said.

A regular subway that isn’t exactly the same as the one we have now

This is the option that makes the most sense to Siemiatycki, who suspects the province’s careful language reflects wanting to have a separate project, rather than something that existing TTC subway trains and track could feed into.

This could be operated by the TTC, city or province, and possibly paid for with the help of the private sector, like the REM.

It would offer more flexibility in terms of procurement and operation as it wouldn’t need to be compatible with the existing subway infrastructure.

“I think what they mean is a line that isn’t interconnected and doesn’t have integrated tracks, or even potentially the same train technologies,” he said.