In recent years significant research activity has been focused towards understanding the processes of railway rail fatigue. A number of serious incidents have been attributed to rail fractures due to rolling contact fatigue cracking. The mechanisms of rail failure are not fully understood but some are related to the relationship between defects and the residual stress field in the rail. Synchrotron X-ray diffraction has been employed to measure residual strain in a worn US rail section. Residual stresses are derived from the measured strains. It is shown that the use of high energy X-radiation can reveal the detailed stress distribution in a steel rail head.