Measurements of the stress‐induced changes in ultrasonic wave speeds in steels typically used in railroad rails are presented. All of the five possible relative changes in wave speeds for a uniaxial state of stress have been determined and agree, to within the limits of accuracy of the measurement, with the second‐order theory of Hughes and Kelly. The third‐order elastic constants are calculated from the acoustoelastic data.Subject Classification: [43]85.20, [43]85.22, [43]35.26.