TORONTO  — A workers union on Friday threatened a strike at one of Canada’s two major freight railroads, only hours after the company’s trains restarted following a potentially devastating stoppage. Trains were expected to keep moving at least through Monday morning.

Canadian National and Canadian Pacific Kansas City Ltd. locked out their workers on Thursday when negotiations over a new labor contract broke down. That resulted in a near total shutdown of freight rail in the country for more than a day, until Canadian National resumed its service.

Trains operated by CPKC remain parked and its workers, who had already been on strike since Thursday, stayed on the picket line Friday.

The government has forced the companies and the union, Teamsters Canada Rail Conference, into arbitration — an order the union is challenging. The union filed its 72-hour strike notice against CN on Friday morning shortly after it announced that it planned to challenge the arbitration order, said union spokesperson Marc-André Gauthier.

It was not clear if the latest action would bring CN trains to a halt again when the strike notice expires 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 that case, CN employees could continue working and CPKC workers could get back on the job while the union continues to challenge the legality of the arbitration order in court. All the parties had a meeting with the CIRB that began Friday morning and continued into the afternoon.

“We do not believe that any of the matters we have been discussing over the last several days are insurmountable and we remain available for discussion in order to resolve this matter without a further work stoppage,” the union said in its letter to CN management.

Perrin Beatty, President and CEO of the Canadian Chamber of Commerce, said the union’s latest actions “will prolong the damage to our economy and jeopardize the wellbeing and livelihoods of Canadians, including union and non-union workers across multiple industries.”

Labour Minister Steven MacKinnon had announced the decision to force the parties into binding arbitration on Thursday afternoon, more than 16 hours after the lockout shut down the railroads, saying the economic risk was too great to allow them to continue.