LAC-MEGANTIC, Quebec — Three more defendants have been added to a class-action lawsuit over the oil train derailment that killed 50 people in Quebec, a team of lawyers announced Thursday.

The lawsuit, originally filed against U.S.-based Montreal, Maine & Atlantic railway and its employees, has been amended to include Irving Oil, World Fuel Services and its subsidiary, Dakota Plains Holdings, the lawyers said in a statement. The motion was filed in Quebec Superior Court.

The lawyers allege the newly named defendants failed to ensure the highly flammable contents in the tankers were properly contained and safely transported.

The disaster occurred when the unattended train came loose and hurtled down a seven-mile (11-kilometer) incline. The train derailed and ignited in Lac-Megantic, near the Maine border. All but one of its 73 cars was carrying oil, and at least five exploded.

The Montreal, Maine & Atlantic train was carrying crude oil sold by Dakota Plains Holdings at the time of the July 6 derailment. Irving Oil runs the refinery in Saint John, New Brunswick, where the oil had been destined to go.

A spokeswoman for Irving Oil said the company sent personnel to the crash site and provided firefighting foam within hours of the disaster.

"We did not own or control the crude oil or its transportation at any time," Carolyn Van der Veen said in an email.

Officials at World Fuel Services and Dakota Plains did not immediately return messages seeking comment.

The lawsuit seeks compensation for the damage caused by the tragedy, but no dollar figure has been publicized.

Another Montreal, Maine & Atlantic train derailed Thursday in a minor incident. One wheel of a cargo train carrying soy came off a track in Farnham, a Quebec town between Montreal and Lac-Megantic, the Transportation Safety Board of Canada said.

There were no injuries, damage and no crossings were blocked, the board said.

Canada's two largest railways, meanwhile, announced Thursday they are strengthening their own safety procedures in the wake of the Lac-Megantic disaster. Canadian Pacific Railway and Canadian National Railway said the tragedy prompted them to review their policies, including brake-setting procedures.

Montreal, Maine & Atlantic railway has also said it is reviewing its safety procedures.