A government watchdog says federal regulators are failing to refer serious safety violations involving freight rail shipments of crude oil for criminal prosecution.

A report by the Department of Transportation's inspector general also says the Federal Railroad Administration doesn't have a complete understanding of the risks involving more than 400,000 tank cars of oil shipped across the country annually. The watchdog says the FRA only looks narrowly at operations in specific regions, not the nation as a whole.

There has been a series of fiery oil train explosions in the U.S. and Canada in recent years, including one just across the border in Lac-Megantic, Quebec, that killed 47 people.

The agency's complex records system also makes it difficult for inspectors to access safety information on rail operations outside their region.

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