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September 1, 2014

Why do train tracks buckle during hot weather and cause derailment

Why do train tracks buckle during hot weather and cause derailment

You see it on the news every summer. Someplace on this good planet Earth, train tracks buckle during a spell of hot weather, causing a train wreck. The reason is well understood. Metals expand when heated; thus, the metal rails in the train tracks got longer. In the old days, there were gaps between the thirty-nine-foot rails that made the familiar clickity-clack sound as the train went over the tracks. A thirty-nine-foot train rail can expand as much as a half inch when the temperature goes from 70 to 100F. The gaps allow the rails to expand.
This process changed a few years ago. Railroad companies now have tracks laid in segments of about fifteen hundred feet or more. This so-called continuous track, or continuous welded rail, eliminates the expansion joints. At the mill, shorter tracks are welded together and carried lying on two flatcars to the intended site. There, the sections are welded together in a thermite fusion-welding process, which involves placing a mixture of aluminum powder and iron oxide in a crucible and igniting them with magnesium. The combustion raises the temperature of the reaction of aluminum powder and iron oxide to more than 5000F, and this mixture is poured into the gap between the two preheated rails to weld them together.

Buckled Tracks, derailment
There are advantages to employing rails that may be a half mile in length instead of the standard thirty-nine feet. The longer joined rails provide a smoother ride, less maintenance, and less fiction than the shorter rails. Trains can travel at a higher speed.
Railroad companies always schedule the process of laying the long sections of track for one of the locally hottest days of the year, so that the rails are not likely to expand further. On cooler days, the rails will be under tension because they will try to contract and pull apart. But the maximum stress on the rail, even if the temperature drops 100F, is less than one-fourth the allowable stress for the steel in a continuous track.
As a matter of curiosity, the distance between the rails is 4 feet, 8.5 inches, believed to be the same distance as the wheels of Roman chariots. Ben Hur was actually ‘riding the rails”.

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