The Canadian National freight train that derailed in Plaster Rock in January, resulting in a large fire and evacuation of the area, had one of its wheels break from "fatigue," says an update issued by investigators from the Transportation Safety Board.

After the derailment, a conductor walked back toward the 19-car derailment and found a broken wheel on the second axle of the 13th car, causing the axle to derail with both wheels inside the track gauge.

derailment - DOT 111 tank car

Among the cars that derailed near Plaster Rock were DOT 111 tank cars. (Transportation Safety Board/Twitter)

"The broken wheel failed due to fatigue," states the update released on Thursday. "A crack initiated at a porosity and travelled under the running surface of the wheel which caused a shattered rim."

The broken wheel was manufactured in 1991 and met the material requirements for that time period, the update states.

Wheels manufactured today undergo an ultrasonic inspection of the tread area to check for areas of porosity, or tiny holes.

"This inspection procedure is carried out to detect and prevent wheels with significant areas of porosity, such as found in the subject wheel, from being placed into service."

The train included 122 cars with three head-end locomotives and a remote locomotive. It was travelling from Toronto to Moncton on Jan. 7 when the derailment occurred.

Of the 19 cars that derailed, nine were carrying crude oil and liquefied petroleum gas.

The two tank cars that were the primary source of the oil that caused the fire were older Class 111 tank cars, built in 1984 and 1996, the investigators stated in the update.

There were no injuries. Approximately 150 people near the accident site were evacuated from the area due to the resulting fire.

While passing a Wayside Inspection System, the crew did receive an alarm. It was following normal procedures and slowing down, but the rail cars began to derail before the train could stop.

The derailment caused the brake pipe to separate and apply the train's emergency brakes.

The investigation continues.