Yesterday, downtown Lynchburg, Va. saw a devastating train derailment in which several tankers hauling crude oil burst into flames. Details are scarce, but here's what we know about these kinds of accidents.

What Causes a Derailment

Train derailments in the United States aren't exactly common, but they happen much more often than you might think. According to the Federal Railroad Administration, more than 300 derailments happened in 2012 alone.

Nearly all derailments are traced back not to negligence or a mechanical problem on the train, but to an issue with the tracks. Just like roads, train tracks are subject to the elements and to the wear and tear of everyday use. Commuter and freight tracks are regularly inspected, and some specific parts of the tracks (such as the fishplate—the clamp that joins the ends of two lengths of metal rails) are examined by rolling computers. But much of this work is done by eye. And human eyes, especially while performing such a tedious job, sometimes make mistakes.

Less likely, but still possible, is human error. Across the U.S., railroads have stringent speed limits in place based on by-the-book engineering specifications delineating how fast a speeding train can take a corner. These speed limits also include a margin of error to eliminate the danger of any unknown forces that could act on a turning train, such as wind or bad track conditions. But mistakes happen.

Could It Have Been the Brakes?

One common misconception is that brake failures often lead to derailments. This is almost never the case. Airbrakes, which all freight trains use, are designed to work the opposite the way the brakes in your car work. It takes energy to turn the brakes off.

In action, airbrakes work like this: Each train car has its own isolated tank of air, which squeezes that car's brakes. When the train is running, these air tanks are always on, trying to clamp the brakes tight. To make the car move, the main brakeline— an independent system—pumps air to counteract those air tanks and release the brakes.

That sounds inefficient, but the idea is that in the event of a malfunction of the main airbrake line—if it breaks open, for example—all the brakes will squeeze against the wheels, violently stopping the train rather than letting it run away. The only way an airbrake failure can lead to a derailment is if the train had been left on the tracks without its emergency brakes on—and for many hours—while the train car's individual air tanks inevitably leaked air. It's a farfetched scenario, but it has happened.

What Causes Crude Tankers to Burst?

It's not yet clear which tankers the Lynchburg train was hauling, but they were likely the most common tank cars in North America, the DOT-111. The DOT-111 is the workhorse of the American railway—if you've seen a freight train, you've seen DOT-111s rolling down the tracks. There are currently more than 240,000 operating in North America.

While the DOT-111 is often used to haul crude, it was not manufactured or engineered to do so. In fact, in 2009 the U.S. National Transportation Safety Board flagged it as unfit to haul ethanol and crude oil, as it's prone to puncture in the event of a crash.

Although there was never been an official government request to do so, DOT-111s made since 2011 have been upgraded with stronger shielding to prevent puncture. Still, most DOT-111 on the rails are the old model and have never been retrofitted. As previous crude oil freighter derailments have proven, hauling crude in these old freight cars is a dangerous and potentially deadly task.

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