A train derailment in Louisiana on Saturday spilled nearly 10,000 gallons of hydrochloric acid and forced more than 3,000 people to evacuate their homes. In the past few weeks, derailments have also occurred in southwestern Pennsylvania, on the Boston T, and on the New York City subway. What causes trains to derail?
Human error or problems with the track. Last year, the Federal Railroad Administration reported 1,876 derailments on U.S. railroads, although the vast majority cause only minor damage. Among those incidents, 46.7 percent were caused by track defects, while another 28.7 percent were caused by human factors. (The cause of Saturday's derailment is still unknown.) The rest were caused by mechanical problems with the train itself, signal failures, and miscellaneous factors ranging from vandalism to snow.
Human-caused derailments are rarely traced back to the proverbial employee asleep at the switch—the FRA reported only one such derailment last year. Instead, accidents more frequently occur because trains are going too fast. Some train wrecks occur when drivers exceed the posted speed limit (19 derailments last year), but, more often, the cars go off the rails because the brakes weren't used correctly.
An object that happens to be sitting on the track isn't likely to derail an oncoming train. A freight train can typically throw a 4,000-pound car out of its way without getting off track—although in 2005 an SUV left on a commuter track caused a fatal derailment in California.
Almost half of all derailments are caused by faulty equipment or track defects. In the United States, a standard gauge (the distance between the two rails) is exactly 56.5 inches, a distance that may or may not be related to the standard width of Roman chariots. If that gauge widens over time or because a crosstie is loose, then the wheels of the train will no longer align with the track. Small cracks in the rail can also result in a train wreck, as in a deadly derailment in Hatfield, England, that caused an overhaul of train safety in Britain.
Over the past 30 years, improvements in the monitoring of track conditions have helped cut the number of derailments by more than two-thirds. (In 1975, U.S. railroads saw 6,328 derailments.) In addition to visual inspections of the track (PDF) twice a week, railroads usually run ultrasonic tests at least once a year to determine whether rails have cracks that can't be seen by humans. Another tool called a "gage restraint measurement system" vehicle measures the strength and geometry of the rails as it passes along the track.
Likewise, improvements in the construction of rails has reduced the number of so-called "rock 'n' roll" derailments. Rock 'n' roll derailments are caused when the interaction between a train and its track creates harmonic motion. At speeds between 12 and 24 miles per hour, a freight car can resonate so that it starts rocking back and forth violently. But with changes in how rails are joined together, rock 'n' roll has become a less frequent source of derailment, accounting for just 34 accidents last year.
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Explainer thanks Michael Logue of the Federal Railroad Administration and Gary Wolf of Rail Sciences Inc.