A CN train derailment in northwestern New Brunswick that sparked a fire and forced 150 people from their homes may have been caused by an "undesired" brake application, according to the Transportation Safety Board of Canada.

Dan Holbrook, TSB manager of western regional operations, told CTV News that a breach in a pipe connected to the air brakes may have triggered an "undesired" emergency brake application, bringing the train to a stop.

The application of the emergency brakes will be the focus of the ongoing TSB investigation, he said.

During a news conference on Wednesday, CN President Claude Mongeau said 17 cars derailed near Wapske, N.B., on Tuesday night. Nine of them were carrying dangerous goods, including crude oil and liquefied petroleum gas. The crude oil was on its way to an Irving Oil refinery in Saint John, he said.

Mongeau told reporters that the impact on the environment, air quality and spill seems to be "very contained and hopefully will be manageable.

"The fire is our first priority at the moment. We have the equipment, we have the people and we have all the procedures in place to deal with it in a safe manner," he said.

Out of the 122 train cars, 66 were loaded, 25 were empty and 31 had residual hazardous material inside, Holbrook said.

Two cars carrying propane gas and one car carrying crude oil are still burning at the site of the derailment, just south of the village of Plaster Rock, Assistant Fire Chief Richard Reid told CTV News Wednesday morning. Fire crews will not be trying to put out the fire, but will let it burn out instead.

The fire prompted an evacuation on Tuesday, forcing about 60 people in the area to leave their homes. By Wednesday, the number of evacuees had climbed to 150.

New Brunswick's Emergency Operations Centre is monitoring smoke from the fire and the province's health department has recommended that residents in the area not drink water that comes from a private well.

CN crews had planned to conduct an aerial surveillance of the site to determine the source and scope of the fire. CN spokesperson Jim Feeny said more information about the blaze is required before crews can go in and start cleaning up the site.

The train – which was heading to Moncton from central Canada -- had only two people on board, the engineer and conductor. Neither sustained any injuries and both have provided statements, Feeny said.

Wapske resident Karen Green told CTV News that the train derailed next to a lumber yard and a pile of lumber caught fire.

Following the derailment there were unconfirmed reports of an explosion. However, Plaster Rock's emergency measures co-ordinator Sharon DeWitt said Tuesday she's not aware of any blast at the scene.

An evacuation centre in nearby Plaster Rock was opened to accommodate the evacuees, but local officials said most people have been staying with friends and family.

As a precautionary measure, a team of volunteers has been dispatched to the area by the Canadian Red Cross to help with the aftermath of the derailment.

With files from The Canadian Press