Canada's transportation safety agency is raising concerns that dangerous crude oil could still be travelling by rail inside misclassified tank cars, despite assurances from the federal government that the problem has been fixed.

In a recent letter to Transport Canada, the Transportation Safety Board said new requirements to test oil don't explicitly address its "variability," including the fact that different products are sometimes blended together before they are shipped.

The letter was sent just days before the TSB issued its final report on the Lac-Mégantic rail tragedy, in which a train loaded with volatile crude oil exploded last summer, killing 47 people and levelling much of the Quebec town. The agency's report, made public last month, found that more than a dozen different factors contributed to the crash, including a failure to apply enough hand brakes, a weak safety culture at the railway and lax regulation by the federal government.

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TSB tests conducted early in the investigation showed that the oil on the train was more volatile than its shipping documents had indicated and it recommended that new measures be taken to ensure shipments are classified accurately. The federal government responded by toughening the rules for testing crude oil samples, including new provisions requiring a shipper to make information about the sampling method they use available to the government upon request.

However, those new regulations "do not explicitly address the variability in the properties of mined gases and liquids, such as petroleum crude oil," the letter from the TSB says. While the properties of manufactured dangerous goods, such as gasoline, are better understood and relatively predictable, the agency warned that crude oil and natural gas can vary from one well to another and in the same well over time.

Oil that comes from different sources may also be blended when it's loaded onto rail cars, the TSB notes. That means crude that was deemed relatively safe during one set of tests – for example, at the time crude is extracted from a well – could be mixed with more dangerous oil when it is loaded onto tank cars, and the overall risk may not be reflected by the original test results. The TSB letter also raises questions about the department's ability to enforce its own classification rules.

Oil is widely known to be flammable, but regulators in Canada did not previously believe it had the potential to explode and cause the kind of destruction it did in Lac-Mégantic. The train that derailed there was carrying light crude from the Bakken formation, which straddles North Dakota, Montana, Saskatchewan and Manitoba. Bakken crude and other light shale oils are now widely believed to be more volatile than conventional oil.

A spokesperson for Transport Canada said there are "strict requirements" under the Transportation of Dangerous Goods Act that compel companies to classify dangerous goods properly. "Testing criteria are harmonized with [United Nations] requirements and are the same as for the U.S.," the spokesperson wrote in an e-mail. She added that the department is working with the crude oil industry, U.S. regulators and Natural Resources Canada to develop standardized tools and processes for crude oil testing.

The American Petroleum Institute recently developed a new set of classification and rail loading standards for its members to approve, which are expected to be made public later this month. Both Transport Canada and the U.S. Pipeline and Hazardous Materials Safety Administration were involved in the process, according to the API, but the new standards would not be enforceable unless regulators chose to adopt them.

In the meantime, some companies are choosing to adopt new testing methods – in addition to those required by federal regulations – to ensure they are accurately measuring the possible dangers of the crude they're extracting or transporting. Producers in North Dakota are also increasingly looking to stabilize the crude before they ship it, in a process that removes the most volatile components from the main product, reducing the potential dangers of shipping it by rail.

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A separate safety advisory from the TSB, which was also issued days before the agency's final report on Lac-Mégantic, warned that some of the problems identified at Montreal, Maine & Atlantic Railway may also exist at other short-line railways. The safety agency said runaway trains occur at a greater rate at short-line railways than larger railways and suggested short-line employees may not always receive the training they need to operate safely.