About 200 kilometres before an ill-fated oil train was left idling on the main track near Lac-Mégantic, Que., Transport Canada conducted a routine inspection and allowed it to proceed. The train carried on through Quebec, carting 72 tank cars of crude bound for the Irving Oil refinery in Saint John.

The inspector didn't report any concerns about the engine in the train's lead locomotive. But The Globe and Mail has learned that part of the federal investigation into the rail disaster has focused on a repair conducted nine months earlier that played a role in a locomotive fire that broke out later that night, setting in motion a series of events that led to the train's derailment and explosion in Lac-Mégantic.

Details of the repair work and its possible connection to the locomotive fire in Nantes, Que., emerged when The Globe spoke with more than half a dozen sources familiar with various aspects of the accident and investigation.

Story continues below advertisement

The revelation sheds new light on a complex series of factors that contributed to the biggest rail disaster in modern Canadian history.

At some point during the train's journey through Quebec's eastern townships and toward Lac-Mégantic, the engine in the lead locomotive began to surge, according to a source, an issue Montreal, Maine and Atlantic Railway engineer Tom Harding deemed serious enough to report to the company.

After he arrived in Nantes, Mr. Harding left the locomotive running and the train's air brakes on, and retired to a hotel for the night. Less than an hour later, firefighters were called to extinguish a blaze in the same locomotive that had caused him trouble. After they left, the train rolled downhill toward Lac-Mégantic, where it jumped the tracks and unleashed a mushroom cloud of burning oil that razed the downtown core and killed 47 people.

The TSB will release its final report on the accident this Tuesday, after one of the agency's most complex and demanding rail investigations. The report is expected to attract significant attention because of the scale of the devastation in Lac-Mégantic and the broader questions the accident has raised about the safety of moving crude oil by rail.

Several legal actions related to the accident are still before the courts, including criminal charges against the railway and three of its employees, and civil suits in the United States and Canada, constraining those with knowledge of the investigation from discussing the matter in public.

However, according to sources, a repair was performed on the locomotive's engine in October, 2012, about nine months before the crash occurred. A source said the material used in the repair lacked the necessary strength and durability and eventually failed.

The locomotive had been built decades ago, and sources indicated that a complete repair would have required the engine to be torn down, a task that would have been both expensive and work-intensive. The failure of the material contributed to a series of other problems, including a damaged piston and engine valves, that ultimately led to a buildup of oil in the engine system and the fire in Nantes, according to a source familiar with the matter.

Story continues below advertisement

While the locomotive was likely using more engine oil than usual in the leadup to the accident, the source said, the problem may not have attracted much attention because locomotives often consume more fuel as they age.

The TSB has already issued recommendations for stronger standards for the rail cars used to transport crude oil, new Emergency Response Assistance Plans for crude and ethanol shipments, and a requirement for railways to conduct a formal route analysis and take other precautions when moving dangerous goods.

In response, Transport Minister Lisa Raitt announced earlier this year that formal emergency plans must be put in place for crude and ethanol shipments, and a new proposal for a tougher tank-car design was recently sent to the rail industry for consultation. Transport Canada has also agreed to speed restrictions, along with added inspections and risk assessments for routes carrying a high volume of dangerous goods.

MM&A and three of its employees, including Mr. Harding, have since been charged with criminal negligence in connection with the crash. Also charged were train operations manager Jean Demaitre and railway traffic controller Richard Labrie.

However, unanswered questions remain about how MM&A was permitted to operate trains staffed by a single employee. Sources familiar with the investigation say it has examined the regulatory environment at the time of the crash, including the role Transport Canada played in allowing MM&A to shift to single-person operations.

Under Transport Canada's rules, companies are supposed to be able to demonstrate that they can run single-person operations safely. While the department did not explicitly grant MM&A permission to proceed with the practice, it did acknowledge the railway's plans to do so, a source familiar with the matter said.

Story continues below advertisement

That source also indicated that MM&A engineers received between 20 minutes and four hours of training before the practice began, and said that, in some cases, training occurred on the same day that the first single-operator trains began running in Quebec.