A giant tunnel-boring machine would allow a Projet Montréal administration to build an entirely new Metro line for under $6 billion, the opposition municipal party said Tuesday at a campaign announcement.

A promise to build the so-called Pink line is the capstone in the party's bid to unseat Équipe Denis Coderre from city hall in next month's municipal election. It would represent the most ambitious expansion of Montreal's Metro system since the late 1980s.

Projet's leader, Valérie Plante, first proposed a new Metro line that would link Montreal North to downtown during last year's race for her party's leadership.

Projet Montréal says building the stations deep underground, as shown in this artist's rendering of one of the new subway stations built in Barcelona, would minimize disruption to street life above. (Projet Montréal) But today's announcement marks the first time Plante has provided more extensive details about the proposal. Her party's chances of success in the Nov. 5 election depend on the project being taken seriously.

In documents released at a technical briefing, the party outlined its belief that innovative new construction techniques could be used to build 21 kilometres of underground track diagonally across the island.

It points to the use of tunnel-boring machines that allowed Barcelona to build a new subway line deep underground, with relatively minor disruption to residents and businesses above the route.

Like Barcelona's Line 9, the Pink line's northbound and southbound tracks would be built one on top of the other.

The proposed line would cover some of the most densely populated and economically disadvantaged neighbourhoods in Montreal, none of which are now served by the Metro system.

Projet Montréal envisions the Pink line running a total of 29 kilometres, connecting Montreal North to Lachine through 29 stations. The segment between downtown and Lachine would be aboveground rail.

All 29 stations would be wheelchair accessible.

Montreal needs 'sustainable solutions' to traffic, Plante says

The party cited three possible sources of funding for the project: the new federal infrastructure bank (capitalized at $35 billion), the Quebec government's $90-billion infrastructure fund and Ottawa's public transit infrastructure fund.

"With the creation of the Canada Infrastructure Bank, the momentum is there," Plante said at the proposed Metro line's unveiling Tuesday, adding that support for the project from Ottawa and Quebec's provincial government will be essential.

Projet Montréal's proposed Pink line would be built between 2021 and 2027. (Projet Montréal) Plante said she knows the proposal is ambitious, and she's aware of the challenges, but the status quo is no longer acceptable.

"The actual network can't support the current amount of traffic without more sustainable solutions," she said.

"We can't build more roads, but we can build more Metro lines."

The Pink line would be built over three phases: a planning phase lasting until 2021, construction of the first 18 stations and 16 kilometres of tunnel between 2021 and 2025, and an additional two years to build the overland component.

Looks good on paper, but costs difficult to predict, says McGill researcher

Meadhbh Maguire, a PhD student at McGill University's School of Urban Planning who specializes in transportation planning, said much of what Projet Montréal is proposing looks good on paper, especially its emphasis on accessibility, but much more detail is required.

"The nature of making an announcement about a project like this in an election campaign is of course that you don't really need to provide a lot of detail and you can stir up a lot of excitement," she said.

Maguire said putting the Pink line through some of the most densely populated and economically disadvantaged neighbourhoods in Montreal is "good political capital" for the party, but it also makes sense from a public transportation perspective.

"If you're going to try and recover the costs of building the line, you need to have high enough ridership," she said. "It's incredibly important."

Whether the Pink line can be built for the projected $5.9 billion is another question that Maguire said was difficult to predict.

Maguire said there are many variables that can end up influencing the price, ranging from how many companies bid for the contract to labour costs.

"That's the kind of thing that can really blow your figures into the unknown as far as [cost] trajectories go," she said.