Forty-seven people were killed in the July 6 derailment and explosion of the train carrying 72 tanker cars of crude oil.
A copy of Montreal, Maine & Atlantic's preliminary investigation, obtained by the Globe & Mail, reportedly claims the broken piston was to blame for the initial fire that happened in Nantes, Que.
Nantes, which is 11 kilometres from of Lac-Mégantic, is where the train was left by engineer Tom Harding the night it derailed.
The disaster was initially blamed on not enough hand brakes being set on the train's 72 cars and five engines.
MM&A is not working with the Transportation Safety Board on its investigation.
The TSB would not comment on or confirm the information about the piston, saying it's too early to draw any conclusions.
"For us, the investigation is ongoing and we're looking at absolutely everything in the sequence of events that night," said spokeswoman Julie Leroux.
"This includes what caused the initial fire when the train was parked in Nantes."
Thomas Walsh, the train engineer's lawyer, said he heard the piston theory a few days ago.
Walsh said this merely confirms that firefighters in the nearby village of Nantes had to put out a fire and turned off the locomotive — which in turn shut off part of the braking system.
He said there's no single factor responsible for the tragedy.
"There's never a circumstance where it's OK to let a train run down a hill, but there are circumstances where possibly the company carrying the volatile substance should impose specific safety conditions — extra conditions," said Walsh.