In this study, a new portable X-ray stress analyzer was designed and manufactured. The purpose of its use is to evaluate the rolling contact fatigue damage in rails for establishing an effective rail maintenance method. An image plate was used in this analyzer for detecting diffracted X-ray beams. The cos alpha method was adopted for X-ray stress analysis from X-ray diffraction data. A fundamental experiment was made first for examning the present measurement system. Residual stresses in rails used in service for six years were also investigated in this study.