After a string of fiery oil train accidents in recent months, the U.S. Department of Transportation (DOT) on Wednesday proposed new rules seeking to address safety concerns over the increasing use of rail to transport crude oil.

The long-promised standards call for a phaseout or retrofit of older rail cars known as DOT-111s, which are known to be vulnerable to leaks and explosions, within two years; new speed and operational restrictions; and a stricter system for testing and classifying mined gases and liquids, among other measures. (Related: "Oil Train Derails in Lynchburg, Virginia.")

"The volume of crude oil being produced and shipped by rail in North America simply did not exist that long ago," wrote Transportation Secretary Anthony Foxx on the DOT's website. "As the facts have changed on the ground so rapidly in the past few years, we must also change how we move this energy." (Related: "N.D. Oil Train Fire Spotlights Risks of Transporting Crude.")

Foxx noted that between 2008 and last year, the number of rail-carloads of oil shipped in the United States jumped by more than 4,300 percent. The jump has been driven by a spike in U.S. oil production from North Dakota's Bakken shale and a shortage of pipeline capacity.

The Department of Transportation also noted that the risks of transporting oil from North Dakota are elevated because the oil tends to travel distances of 1,000 miles (1,600 kilometers) or more to coastal refineries. (Related: "Oil Train Revival: Booming North Dakota Relies on Rail to Deliver Its Crude.")

Also at issue is the particular composition of crude coming out of the Bakken shale. In a report accompanying the rulemaking announcement, the Department of Transportation noted that oil coming out of the Bakken shale "has a higher gas content, higher vapor pressure, lower flash point and boiling point and thus a higher degree of volatility than most other crudes in the U.S., which correlates to increased ignitability and flammability." (Related: "Illinois Village Leads Charge for Tougher Train Rules.")

It was an oil train filled with Bakken crude that led to tragedy last summer in the town of Lac-Mégantic, Quebec, when a train comprising 72 tanker cars derailed in the town center. The ensuing explosion leveled the town and killed 47 people. Subsequent crashes have occurred in Lynchburg, Virginia; Casselton, North Dakota; Aliceville, Alabama; and other locations in the United States. (Related: "Oil Train Crash Probe Raises Five Key Issues on Cause" and "Oil Train Tragedy in Canada Spotlights Rising Crude Transport by Rail.")

Canada has also announced new oil train safety rules aimed at improving railway accountability and calling for a phaseout of the older DOT-111 cars.

The rail industry issued a statement saying it was still studying the details but welcomed the regulatory action, which will be opened to a 60-day public comment period before it is finalized. "This long-anticipated rulemaking from DOT provides a much-needed pathway for enhancing the safe movement of flammable liquids in the U.S.," Association of American Railroads President and CEO Edward Hamberger said.

Railroads typically do not own the tank cars used for oil transport—leasing companies contract the cars out to the oil industry. The American Petroleum Institute said Wednesday that it was reviewing the new rule, but it also labeled as "speculation" the DOT's assertion that Bakken crude is more flammable than other crudes.

"The best science and data do not support recent speculation that crude oil from the Bakken presents greater than normal transportation risks," said API President and CEO Jack Gerard. "Multiple studies have shown that Bakken crude is similar to other crudes."

The Department of Transportation report said that the agency relied on "months of unannounced inspections, testing, and analysis" to arrive at its conclusions about the qualities of Bakken crude. (Related: "Eight Steps for Safer Oil Trains Eyed by U.S. Safety Officials.")

Karen Darch, village president of Barrington, Illinois, a Chicago suburb regularly traversed by oil train traffic that has led a push for tougher rail regulations, said Wednesday that although she was still reviewing the DOT proposal, she was concerned to see that only trains carrying 20 carloads or more of flammable liquids would fall under the new standards. That potentially leaves room for mishaps with trains carrying smaller loads, she said. She also questioned the efficacy of speed limits.

"Our focus has been on getting the material in a safer car because even at low speeds, you can have a derailment. And the best defense is having the car hold up in a derailment," Darch said. "We think the enhanced standards that they're proposing for the tank cars are the best solution, and we would like to see that universally adopted for train cars carrying flammable hazardous material."