This paper is the second report on axial stress measurements of a long welded rail making use of an acoustoelasticity effect of V-reflection longitudinal waves. The cause of accuracy deterioration in the field measurement, which was a problem in the previous report, was investigated and a newly improved axial stress measurement system was developed. Axial stresses of long welded rails which were constructed in winter or in summer seasons were measured using the new system. The monthly variation of the axial stress distribution in each rail was investigated for three months. As a result, the non-destructive measurement of the rail axial stress, within a measurement error of 5 MPa in the field measurement, was achieved. It was confirmed that the long welded rails which were constructed in both seasons are now being firmly fixed by both tie and ballast in the immobility section. Furthermore, it was clearly shown that the longitudinal resistance of the ballast in the movable section is about 1/3 of that commonly believed, and that the length of the movable section is gradually extended for several months after construction.

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