Investigators with the Montreal, Maine & Atlantic Railway say the damaged part allowed unburned fuel to seep throughout the locomotive's engine, causing smoke and sparks, The (Toronto) Globe and Mail reported Friday.
The fire was put out and fire crews turned off the engine, deactivating the locomotive's hydraulic brakes, officials say. The train's hand brakes were not set.
The train rolled down a hill and smashed into the Lac-Megantic business district. The cargo of crude oil exploded, killed 47 people and destroying much of the downtown.
The Transportation Safety Board, which is conducting its own investigation, has called for regulations spelling out how many hand brakes must be set to back up the hydraulic brakes.
The TSB's final report on the crash is not expected for several months.