The National Transportation Safety Board suspects that yesterday’s Amtrak derailment near Washington was caused by a “heat kink” in the track. What’s a heat kink?
Think back to an axiom taught in countless fourth-grade science classes: Metal expands when heated. As temperature increases, metal atoms gain energy and begin to move more briskly. These active atoms require more space and push adjacent atoms farther away. The end result is a chunk of metal that bloats and bends. This is why Mom submerged jars with hard-to-unscrew lids in boiling water—the atoms get energized, the lid expands, and the Miracle Whip suddenly becomes accessible.
Railroad tracks are no different. When air temperatures approach 100 degrees, small patches of track can bow out several inches, sometimes even a foot or more. Sunlight provides the necessary heat, and metal fatigue and design imperfections make the tracks more malleable.Heat kinks primarily affect “welded” tracks, like the one near Washington, which use continuous bars of metal as a means of reducing noise and improving passenger comfort. Older “jointed” tracks feature small gaps that can accommodate moderate expansion—much like metal bridges, which are typically studded with mesh-covered cracks to prevent hot-weather buckling.
A heat kink does not make derailment inevitable. Speed, weight, and track curvature combine to determine a train’s fate. Slower choo-choos are less apt to succumb to heat kinks; after a 1998 accident in Texas, Union Pacific Railroad mandated that trains not exceed 10 mph through areas with known or suspected kinks.
That rule wouldn’t have helped the ill-fated Amtrak train since neither inspectors nor other trains had noticed any warping prior to the crash. Kinks can develop in a matter of hours, so even twice-daily visual inspections are sometimes insufficient. (The last inspection before yesterday’s derailment took place the previous evening; a freighter passing over the same spot 45 minutes beforehand reported no irregularities.) One possible solution is fiber-optic cable, which could be affixed to rails and sound an alarm whenever flaws arise. Researchers at the University of Illinois, Urbana-Champaign, began developing such a system in the mid-1990s; apparently it hasn’t made its way to the East Coast yet.
