The measurement of rail longitudinal force in CWR (Continuously Welded Rail) is important so as to avoid the rail failure in winter and the track bucking in summer. The effectiveness of rail lifting method for it in winter was demonstrated on the ballasted track experimentally and theoretically. As the application of it to the slab track is conceived in Japan, theoretical analyses are proceeded for it. As a first approach, the elastically supported rail with single stiffness outsides of a released section for rail lifting at the center of it is analyzed. The ruff characteristics of the measurement are given with it. However, as the responses of rail fastening devices are quite different for the amount and direction of force in the section of elastically supported rail, the second model with two kinds of stiffness for this section is analyzed. As astonishment, the remarkable decrease of force to the first rail fastening device from the boundary of the released section is given. It is sure that such a decrease in displacement is brought by the large stiffness in the outer section of rail elastic support. With these facts depending on more plausible analysis, all the difficulties for the measurement are excluded. The linearity of lifting force to the rail tension is guaranteed.
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