The Transportation Safety Board of Canada says a "sudden and catastrophic failure" of a rail caused a fiery train derailment in Saskatchewan.
The board says a rail that broke due to an undetected defect led to the derailment of a Canadian National Railway freight train in Clair, east of Saskatoon, in October 2014.
Lead investigator Rob Johnston says regular ultrasonic track tests are required once a year and CN had inspected the section seven times the year before the accident. But the surface of the rail masked the defect, he said.
"CN far exceeded the regulatory requirement because they did recognize that there were issues with certain areas of this track," Johnston said at a news conference on Wednesday.
"Unfortunately, this one kind of just fell through the cracks."
The rail section was built in 1959.
CN said at the time that the train was going within the speed limit of 40 km/h when it derailed and that the stretch of track had been inspected just a day before.
In an email Wednesday to The Canadian Press, CN said preventative measures have been taken and investments made to annual maintenance since 2014.
"Our safety statistics have considerably improved since then and we continue to push forward," the company said.
"We also actively implement leading-edge technologies to mitigate risk through increased monitoring of track and equipment, as well as ongoing data analysis to identify potential risks before accidents happen."
The 100-car train was heading to Edmonton from Winnipeg when 26 cars slid off the tracks. Two tanker cars loaded with petroleum distillates leaked.
During cleanup, Johnston said, two CN emergency responders were burning off the contents of one of the breached tankers when there was a flash fire. Vapours in the tank ignited and sent a large fireball towards them. No one was hurt.
Johnston said CN emergency responders were probably tired and didn't recognize the risk of flares.
"Usually it's done safely. In this case, I think they were caught a little bit off guard."
The email from the railway said two CN emergency responders, both industrial firefighters wearing personal protective equipment, produced a controlled flash fire that burned for two seconds to eliminate flammable vapours and allow cleanup work to safely continue. It said this was done in co-ordination with local fire and regulatory officials on site.
The TSB report also said the chief of the Wadena volunteer fire department, who was designated the incident commander, and the assistant fire chief were experienced firefighters, but they had no experience with that type of response.
It was the Wadena fire department's first derailment involving dangerous goods and it was not properly equipped to respond, the report said.
Fifty residents nearby were forced to leave their homes, but there were no injuries.
The TSB said it has investigated six similar occurrences over the last decade in which rail breaks were either the primary cause or a contributing factor to a derailment.
Johnston said he knows communities have concerns about rail lines but, overall, they are safe.
"The railway doesn't want to go to work and have something like this happen, so they do take these things very seriously."
The tankers involved were the same type as those in the Lac-Megantic disaster in which a train derailment killed 47 people in the Quebec community in 2013.
As of Nov. 1, railway companies can no longer use that older kind of rail tanker to transport crude oil or other dangerous goods in Canada.
Regulations announced in May 2015 require all new tank cars built for the transportation of flammable liquids be constructed using thicker and more impact-resistant steel.