North Dakota's oil boom is a big, probably underappreciated story. As the rest of the United States tumbled into the Great Recession in 2008, sending national unemployment as high as 10 percent, North Dakota's jobless rate peaked at 4.2 percent in 2009; as of October 2013, it was at 2.7 percent, the nation's lowest.

North Dakota's good fortune is largely attributed to the light, sweet crude being coaxed and bullied out of the Bakken shale formation in the northwestern part of the state. The Bakken crude has made North Dakota the U.S.'s second-largest oil-producing state, after Texas, and helped boost U.S. oil production to its highest level since 1988. Next year, the U.S. might well pass Saudi Arabia as the world's top oil supplier.

On Thursday, the Transportation Department issued a warning about the Bakken oil:

The Pipeline and Hazardous Materials Safety Administration (PHMSA) is issuing this safety alert to notify the general public, emergency responders, and shippers and carriers that recent derailments and resulting fires indicate that the type of crude oil being transported from the Bakken region may be more flammable than traditional heavy crude oil. [PHMSA]

This bland-sounding advisory was enough to send the shares of Bakken oil producers and transporters down a notch. There had been trouble with Bakken oil before, most fatally in July 2013, when a runaway train carrying the crude derailed and exploded in Lac-Mégantic, Quebec, killing 47 people. Another Bakken oil train derailed and exploded in Alabama in November, with no casualties.

Still, in December North Dakota regulators were preparing to commission a study to prove that Bakken oil is not dangerously explosive to move by rail. Then, on December 30, a 106-car BNSF train carrying Bakken crude crashed into a derailed grain train near Casselton, N.D., 20 miles west of Fargo:

The massive explosion prompted an evacuation of Casselton and a decision to drop the study. CBS News has a longer report on the explosion:

What makes Bakken oil more flammable than other oil, like the heavy crude harvested from Canada's tar sands? Basically, it contains more natural gas, making its vapor combustible at a much lower temperature. More specifically, Bakken crude tends to have sizable amounts of easily ignitable propane and butane, Zak Mortensen at oil-quality analysis firm Inspectorate America Corp. tells Bloomberg News.

The volatility of North Dakota's black gold wasn't exactly a secret. "It doesn't take a rocket scientist to figure out that Bakken oil is a high-quality crude with a lower flash point," the temperature at which oil vapors ignite, North Dakota Petroleum Council president Ron Ness tells the Associated Press. "That's what makes it a desired commodity for all these coastal refineries."

But that doesn't solve the question of what to do with the information. Rail shipments of oil have skyrocketed in the past four years, from 10,000 tanker cars in 2009 to an estimated 400,000 in 2013. Oil and natural gas are now the fastest-growing category of rail cargo, the Association of American Railroads reported earlier this week. Much of the growth is from North Dakota, which sends more than 60 percent of its crude to the East Coast, Pacific Northwest, and Gulf Coast by rail.

Nobody in the industry seems to want this to change. "The railroads obviously want this business," rail analyst Jason Seidl tells Bloomberg News. "It's going to continue to grow." And oil producers are "going to keep railing to the East Coast," adds energy consultant John Auers. "They're going to keep moving Bakken to the Pacific Northwest. There's no other option."

There is, of course, another option: Pipelines. But pipelines can and do rupture, and Bakken crude won't become any less explosive just because it isn't being carried by rail. Also, the only proposed pipeline to pass through North Dakota — Keystone XL — goes only south, toward the Gulf Coast.

The Casselton crash has prompted the National Transportation Safety Board to begin the nation's first major examination of the safety of carrying oil and gas by rail. But there's no lack of ideas to make things safer. An obvious one is to route petroleum-carrying rail cars around cities and towns, not through them. Another option would be to make oil trains move at slower speeds.

The most likely change, though, will be to make rail lines use stronger tanker cars to transport Bakken crude and other volatile petroleum products. We've known about dangerous flaws in the most commonly used current tanker designs for more than two decades, Larry Bierlein at the Association of Hazmat Shippers tells the AP. And the NTSB has already issued guidelines for more resilient tankers. The Transportation Department just has to adopt them.

This will increase the cost of moving oil by rail somewhat. Given the proximity and price advantages of the Bakken oil, few analyst expect that the oil and rail industries won't pay.