Prime Minister Justin Trudeau says his government's $1.28-billion investment in Montreal's light-rail system will ease the burden on commuters and contribute to the fight against climate change.

The electric-rail project, which is led by the Caisse de dépôt et placement du Québec and also has funding from the Quebec government, will link Montreal's downtown to its suburbs and international airport.

Trudeau confirmed his government will fund the "ambitious" plan, known by its French acronym REM, on Thursday at Montreal's Central Station.

"In addition to making it quicker and easier for millions of Quebec residents to get around, the REM will reduce the number of cars on the roads, help ease traffic and make the air cleaner," Trudeau said.

"With this historic investment, we're taking a huge step in the right direction."

Quebec Premier Philippe Couillard, Montreal Mayor Denis Coderre and the head of the Caisse, Michael Sabia, were also on the hand for the announcement.

The funding comes after months of negotiations — and questions about whether Ottawa would back the plan.

The Trudeau government made no mention of funding the project in their spring budget, prompting concern from Couillard.

On Thursday, Trudeau said the investment is consistent with his government's broader commitment to improving public transit across the country.

The price tag for the 67-kilometre rail line, which would link downtown with the South Shore and the West Island, has risen to $6 billion in order to add more stations in the city's downtown area.

Quebec has already committed around $1.3 billion to the project. The province's pension fund manager, the Caisse, has pledged close to $3 billion.

Construction to begin this fall, Couillard says

The LRT project now includes 27 stations. The first trains are expected to run in 2020, though there have been questions about the timeline.

Couillard said Thursday that construction work will begin this fall and the first trains will be running by the end of 2020.

He expects Bill 137, legislation aimed at cementing the agreement between the government and the Caisse, to pass in the fall.

Montreal's light-rail system will include a total of 27 stations. (Caisse de dépôt et placement du Québec)

For his part, Sabia said he sees "absolutely no reason" why the project can't be on time and on budget, adding that it's not "a law of physics" that a construction project in Quebec should be subject to delays.

"Delivering it on time and delivering it on budget can and should be a source of pride for people here in Quebec," he said.

The investment will come in the form of a grant-type investment via Ottawa's infrastructure-funding agreement with Quebec.

It could eventually be funded through the Trudeau government's proposed infrastructure bank, the prime minister said Thursday.

When asked whether the long-discussed extension of the Montreal Metro's Blue line would eventually get federal support, Trudeau didn't rule out the possibility in future.

But he said it should be up to local governments — at the municipal and provincial level — to determine what gets funding.