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Abstract:Based on the results of wheel slide tests and an inspection of wheels, the root cause of tread buildup (TBU) was identified as
wheel slide caused by excessive brake force. During the tests, TBU accumulated to the greatest heights under dry conditions, at
longer slide distances, and under heavier axle loads. Train speeds between 20 and 30 mph appeared to increase TBU height.
Chemical analysis of TBU samples indicated that the source of the material is likely a combination of wheel and rail steel. This
finding reinforces the conclusion that wheel slides cause TBU. A microstructural evaluation of several TBU samples found no
martensite, a microstructure that results when hot steel is rapidly cooled. A relatively slow conductive heat transfer rate from the
irregular contact between the hot TBU and the cooler wheel likely does not provide sufficiently rapid cooling for martensite
formation.
The railroad industry is currently poised to reduce TBU through improved brake system performance as a result of handbrake
training for train crews, handbrake design improvements, an improved airbrake test, and the increased use of data from wayside
detectors to identify cars with brake problems. Although TBU removals are decreasing, TBU-related accidents do not show a
clear trend as a percentage of all wheel-related accidents.
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