Field tests and accompanying data analysis to characterize the stable and hunting behavior of freight cars are discussed. These tests confirmed the fundamentally nonlinear nature of the conventional freight car. The hunting performance of an open hopper car is described by speed ranges in which intermittent hunting occurs. At speeds above the intermittent hunting speed ranges, hunting always occurs, and at speeds below these ranges, hunting does not occur. Results of evaluating the stability of the freight car in terms of the natural frequency and damping ratio of the least-damped vibration mode are presented. Anomalies in these results indicate that the damping ratio may not be an adequate stability measure for this nonlinear system. Root-mean-square values of significant vehicle motions are presented for the entire range of vehicle test speeds.

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