Canadian National Railway faced picket lines last week, but the union said it does not plan this new job action to be as disruptive as the strike that hamstrung Canada's largest railway in February.

In response, CN Rail told the United Transportation Union (UTU) it would lock out workers in the areas where picket lines have been set up, including Vancouver, Canada's largest port.

Canada's labor minister warned of possible Conservative government intervention if the latest labor action was seen as too disruptive to the economy. The opposition Liberals called for the House of Commons to end its Easter break early to deal with the dispute.

The union that represents 2,800 Canadian National workers launched rotating strikes late on Tuesday after workers overwhelmingly voted down the tentative contract deal that had ended February's 15-day walkout.

Union officials said they were still waiting to hear from CN Rail on a return to the bargaining table. A company spokesman has said it is willing to resume talks, but has not set a timetable on when that will happen.

Picketing by the conductors, brakemen and switchyard employees initially began in Vancouver but soon spread to three switching yards in Ontario, including Oakville.

The picketing at each location is expected to last for only a short time, but could resume later in the week. February's walkout was a general strike by CN's 2,800 UTU members.

"We're not planning a full shutdown of the railroad," said Bob Sharpe, a UTU vice-president.

But the company said its lockouts would remain in place until the dispute is resolved.

"The rational is that CN is a scheduled railroad, and we can't run a scheduled freight operation without scheduled manpower," spokesman Mark Hallman said.

The sides are at odds over wages, but many employees have said they are more concerned about issues such as work rules and rest breaks.

February's walkout caused layoffs at CN customers in the auto and forestry industries and disrupted grain exports through Vancouver on both Canadian National and smaller rival Canadian Pacific Railway.

The Canadian Wheat Board warned of new problems even if the strike is more limited in scope.

"This is a really bad time, from a farmer perspective, for there to be any delays in grain movement because they're in the pre-seeding period," said Maureen Fitzhenry of the CWB, Canada's largest grain shipper.

Canadian National is using management crews to replace striking workers, but Chief Executive Hunter Harrison warned customers on Tuesday there would be service delays.

Labor Minister Jean-Pierre Blackburn urged the sides to resume negotiations, and said he was concerned the dispute could disrupt the economy.

The federal government had threatened to order an end to February's walkout, but the back-to-work legislation was put on hold during the contract vote.

Blackburn said he could revive the bill quickly if needed, although Parliament is not scheduled to return until April 16. Federal officials have said a decision on whether to revive the legislation will depend on the level of disruption.

"The back-to-work legislation remains on the order paper ... ready to be passed if necessary," Blackburn said in a statement.

The Conservatives would need unanimous consent of the three opposition parties to push the bill quickly through the legislative process, and the pro-union New Democratic Party is unlikely to support such a measure.

The main opposition Liberal Party said rail service was vital, and it wants the government to call lawmakers back to Ottawa by Friday to begin work on legislation.

The strike does not involve CN's operations in the United States, and the union has said it has no plans to disrupt commuter passenger service in Toronto or Montreal. Via Rail passenger trains are also not involved. (Reuters)