The board said the derailment north of Plaster Rock, N.B., was caused by a rare, undetected defect inside a wheel on a hopper car.
Though no one was injured and the environmental impact was minimal, the board's senior regional investigator said the accident — coming six months after a derailment in Lac Megantic, Que., that claimed 47 lives — again highlighted the vulnerability of older Class 111 tank cars.
Don Ross told a news conference that new regulations for tank cars introduced by Transport Canada in April 2014 are a positive development, but the board is concerned with the length of time it will take to implement the changes.
"Given the lengthy transition period put forward by Transport Canada and U.S. regulators, risks will remain in the system," he said.
Ross said some older Class 111 tank cars will continue to carry flammable liquids until 2025. This type of tank car was among those that derailed and caught fire near Plaster Rock on Jan. 7, 2014 and on July 6, 2013 in Lac Megantic.
"We're concerned about the 2025 timeline," said Ross. "We're in favour of a more aggressive timeline. We think the industry can respond to that."
CN issued a statement Friday noting that the board's report found the railway's emergency response in Plaster Rock was effective. As well, CN spokesman Jim Feeny said the company developed new communications protocols at the time, which have since been adopted for all major incidents.
The board's final report on the derailment says the Canadian National freight train was travelling from Toronto to Moncton, N.B., when the crew received an alarm from a trackside warning system.
The independent agency says that as the crew slowed the 122-car train to a stop just north of Plaster Rock at 6:47 p.m., 19 cars and a so-called remote locomotive in the middle of the train derailed.
Four Class 112 tank cars were carrying a liquefied petroleum gas and five Class 111 tank cars were carrying crude oil.
Two of the Class 111 cars — built in 1984 and 1996 — were similar to those in the Lac Megantic disaster, Ross said. Both of them were ruptured in the crash and leaked 230,000 litres of oil.
The resulting fire grew so large and intense that emergency officials evacuated homes in a 1.6-kilometre radius.
The board's report says the derailment and other derailments in the past two years in Alabama, North Dakota and Virginia have highlighted the fact that Class 111 cars are vulnerable to accident damage.
"If Class 111 tank cars that do not meet enhanced protection standards transport flammable liquids, there is an ongoing risk of product loss and significant damage to persons, property and the environment when these cars are involved in accidents," concludes the board's findings.
The board found the cause of the derailment was a shattered wheel rim caused by a crack in the wheel's running surface.
Investigators said the crack was likely caused by a void within the steel that appeared when the wheel was made in the United States 1991.
Ross said the wheel was inspected using ultrasonic equipment when it was made and again in 2006, but those records no longer exist. He said it wasn't until 2010 that wheel shops were required to keep the results from such tests for 10 years.
According to the board, from 2010 to 2014, the average number of accidents in Canada caused by broken wheels was three per year.
"However, when focusing on wheel failures due to shattered rims, a subgroup of wheel failure, it is clear that the risks associated with this type of failure have been steadily declining as a result of the action taken by the industry," the report says.