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A thermite weld failure resulted in a Canadian Pacific train derailment in Manitoba in August 2017, the Transportation Safety Board of Canada (TSB) announced yesterday.On Aug. 11, 2017, a CP unit train was heading north on the Emerson Subdivision when 22 covered hopper cars loaded with corn derailed while exiting a bridge over the Roseau River near Dominion City, Manitoba. The incident resulted in no injuries and no product or rolling stock entered the waterway, TSB officials said in a press release.The agency's investigation determined that the train derailed when a thermite weld, which joins two pieces of rail, failed under the train. The thermite weld fracture displayed "finning," which occurred when molten metal leaked from the casting mold and did not fully fuse with the rail, TSB officials said.The thermite welding process had been completed in 2008. Also, due to the presence of multiple persistent and recurring geometry defects and track conditions, the rail in the vicinity of the thermite weld had been experiencing higher stresses than during normal train operations. With the repeated tensile forces due to normal passing of each wheel of a train, the finning became the origin site of the fracture that led to the brittle failure of the thermite weld.Although CP had been monitoring the priority surface defects in accordance with federal rules and requirements, the thermite weld failed before the priority defects reached a level that required immediate repair.If track geometry priority defects are not identified, monitored and mitigated in a timely manner, further track structure degradation can occur, increasing the risk of derailments, TSB officials said.Since the accident, CP completed a rail replacement program on the subdivision, installing about 5,000 feet of continuous-welded rail.