TIMEOUT FOR TECH
FIGURE 1. Photograph of a bright June sun. Summer heat and winter cold can induce forces in railroad rail large enough to cause track failures. (Courtesy of Gary T. Fry.)
UNDERSTANDING
THERMAL FORCES IN RAIL How temperature variations can cause sun kinks and pull-aparts
elcome to “Timeout for Tech with Gary T. Fry, Ph.D., P.E.” Each month in this series, we examine a technology topic that professionals in the railway industry have asked to learn more about. This month we discuss how temperature variations can cause forces in railroad rail, even when trains are not present. Figure 1 is a photograph of the sun on a
28 Railway Age // July 2022
bright day in June. How is it that the heat of a summer day can cause several yards of railroad track to suddenly shift sideways out of alignment by a couple feet or more? Conversely, why does a rail suddenly snap in two on a cold winter day? Several scientific principles are in play to answer these questions, but one stands out as the most significant. Rail steel, like most metal materials, expands when heated and contracts when cooled. There’s a fun way to think about this
phenomenon. Being connected by rail lines, shouldn’t Chicago Union Station and St. Louis Union Station be pulled closer to one another in the winter and pushed farther apart in the summer? For example, over the 260-mile distance between the two stations (as the crow flies), rail laid stressfree at 65-degrees Fahrenheit will be 310 feet longer on a 100-degree summer afternoon and 490 feet shorter on a 10-degree winter night. That’s a swing of 800 feet! Setting railwayage.com
Gary Fry
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By Gary T. Fry, Ph.D., P.E., Vice President, Fry Technical Services, Inc.