The part of rail joint becomes the cause of the disadvantage points such as ride discomfort, undesired sound and track maintenance, etc. For restraining those points, the long rail made by welding the standard length rails has been developed. However, the occurrence of buckling is pointed out. The problem of the overhang or breakage by buckling is caused by the axial force produced in the rail. The stress produced by the force is called thermal stress. The stress is brought out by the weather conditions or the friction heat between wheel and rail. Though some evaluation techniques for the buckling management are produced, the development of the technique with non-destructive testing which quantitatively evaluates the force at the good accuracy and reduces the labor for measurement worker is desired. In this study, the relationship between the increase in compressive stress and the behaviors of characteristics of ultrasonic echo was researched. The stress was increased in 10 MPa interval, and the echo at that time was detected. From that experimental results, it was found that the frequency analysis results of detected echoes fluctuate with the increases in the stress. Especially, it became clear that there is the strong correlation between the ratio of the maximum wavelet coefficients (RMWC) of a wavelet transform result (i.e. a kind of time-frequency analysis) of each echo and applied stress. Therefore, it was possible to show the possibility of comparatively accurately predicting the force by comparing the transition of the RMWC to that of the initial state.
