This study and verification are based on the Brillouin frequency shift (BFS), which is related to the strain and temperature changes of a single-mode fiber, because such a shifted frequency can be quantitatively measured and converted to strain and temperature differences. We explain the installation of a Brillouin distributed fiber sensing system (DFOS) on an actual operating railway to measure the temperature and strain of the rail. In addition, the measured data were calculated and analyzed, revealing the geometric irregularity of the tested rail and the location of the abnormality. We obtained a temperature difference of 12.1 °C between the temperature distribution of the measured rail and the atmospheric temperature, and there was a 1.5 h delay between the two. We also obtained rail irregularities ranging from −0.3 to +0.4 mm by calculating the slight strain difference of the rail in this test.