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Firefighters in Nantes, Que., inspect a row of none oil tankers sitting on a railway siding there on July 10, 2013. This is where the ill-fated train that crashed and burned in Lac-Mégantic originated.PETER POWER/The Globe and Mail

The crash of a runaway train in Quebec is calling attention to the crucial role of hand brakes in keeping tank cars out of harm's way.

The estimated number of hand brakes that must be applied on rail cars depends on factors such as the grade on which the train is parked and the length and weight of the train. Too few spells disaster.

It is unclear how many hand brakes the engineer at Montreal, Maine & Atlantic Railway should have set manually to secure the train. But industry guidelines suggest that safety would have been assured if a much greater number of those brakes were activated late Friday night by the MM&A engineer.

No comprehensive rules dictate how many hand brakes to set, but railway workers have special instructions from their companies to guide them for different track locations. The steeper the grade, the more hand brakes they are required to apply. Whatever the estimated minimum number of hand brakes that might be needed to secure a train, engineers are supposed to conduct a brake effectiveness test by pushing and pulling the tank cars with locomotive power to double-check whether the train moves.

Shaped like a large steering wheel on each tank car, the hand brakes must be manually turned or cranked to be applied, so it can be time-consuming. The MM&A train had a single-employee crew. The engineer ended his shift late on Friday, parking the train for the next person to take over on Saturday.

It had 72 tank cars of oil, one buffer car and five locomotives. Based on safety guidelines and the terrain, about 40 per cent of the hand brakes should have been activated to be on the cautious side, according to preliminary estimates from industry, union and safety experts.

Earlier this week, MM&A said that the hand brakes on five locomotives and 11 tank cars were supposed to be activated, or 20.5 per cent of the total train, to prevent what the industry calls "unintended train movement." It has yet to be determined whether that level of brake security would have been sufficient. The engineer did not apply a total of 16 hand brakes as the company had said earlier: MM&A chairman Edward Burkhardt told reporters on Wednesday that the engineer failed to apply an adequate number of brakes.

Even before Mr. Burkhardt's startling revelation, officials at the Transportation Safety Board of Canada were already reviewing whether hand brakes on the MM&A train were manually turned on properly, and if they were activated, the question remains how many of the individual tank cars were braked. As well, investigators have been looking into the air brakes, which can also be used in securing a train, and are controlled by air pressure.

But even if the air brakes on one key locomotive were somehow disengaged, as Mr. Burkhardt claimed at one point, the application of enough hand brakes should have prevented the train from rolling down a 1.2-degree grade toward Lac-Mégantic.

Safety board investigators will examine whether the MM&A engineer, Tom Harding, properly carried out the brake effectiveness test late on Friday night. The TSB is also investigating the terrain where the train was supposed to be locked down and the circumstances of the fire that broke out in one of the locomotives early Saturday.