TORONTO (AP) — A Canadian workers union issued a 72-hour strike notice to one of Canada’s two major freight railroads Friday, only hours after the company's trains had begun rolling again following a potentially devastating stoppage. Trains were expected to keep moving at least through Monday morning.

The Teamsters Canada Rail Conference filed its strike notice against Canadian National shortly after it announced that it planned to challenge a government order forcing it into arbitration with both CN and Canadian Pacific Kansas City Ltd. to end a lockout, said union spokesperson Marc-André Gauthier.

It was not clear if the announcement would bring trains to a halt again when the 72 hours are up at 10 a.m. Monday. The whole matter might be decided before then by the Canada Industrial Relations Board, which is overseeing the arbitration, in which case employees could return to work even as the union continues to challenge the legality of the arbitration order in court. All the parties were scheduled to meet with the CIRB Friday morning.

Labour Minister Steven MacKinnon had announced the decision to force the parties into binding arbitration on Thursday afternoon, about 16 hours after a lockout shut down the railroads, saying the economic risk was too great to allow them to continue. He had declined to order arbitration a week ago, saying he hoped that negotiations between the companies and the union on a new contract would succeed.

“This is not about disobeying the minister’s order. It’s about exercising our right,” Teamsters Canada President Francois Laporte said Friday in announcing the strike. “We will exercise our right within the legal framework."

Canadian National trains had begun rolling at 7 a.m. across Canada, said CN spokesperson Jonathan Abecassis. The development initially appeared to at least partially end a work stoppage that threatened to wreak havoc on the economies of Canada and the United States. Both countries, across all industries, rely on railroads to deliver their raw materials and finished products.

Trains operated by CPKC had remained parked and workers continued to picket there Friday morning as the union prepared its challenge to the arbitration order.

Both railroads had said they were committed to getting their trains running again as soon as possible and would follow the CIRB's ruling, but it wasn’t immediately clear how quickly that would come.

“While CN is focused on its recovery plan and powering the economy, Teamsters are focused on getting back to the picket line and holding the North American economy hostage to their demands,” railroad spokesman Jonathan Abecassis said following the union's announcement.

A spokesman for MacKinnon declined to comment.

All of Canada’s freight handled by rail — worth more than $1 billion Canadian (US$730 million) a day and adding up to more than 375 million tons of freight last year — stopped Thursday along with rail shipments crossing the U.S. border. About 30,000 commuters in Canada were also affected because their trains use CPKC’s lines. CPKC and CN’s trains continued operating in the U.S. and Mexico during the lockout.

Billions of dollars of goods move between Canada and the U.S. via rail each month, according to the U.S. Department of Transportation.

“There are a lot of goods and services shipped across borders," Sean O’Brien, President of the International Brotherhood of Teamsters, said at a rally in Calgary, Alberta, on Friday. "If this company chooses to continue its bad behavior then it is going to have an impact. ... They’ve got a lot of decisions they need to make. And they need to make the most important decision: Reward these workers with what they’ve earned and also don’t try to diminish safety just so they need to feed their bottom lines.”

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Funk reported from Omaha, Nebraska.