Rail transportation is one of the most important and efficient forms of transportation. Large thermal strain can develop in the rail steel due to extreme climatic conditions resulting in safety related issues. We carried out a thermal-strain monitoring test on the rail specimen over a large temperature range from −40 ${}^{\circ }$ C to +50 ${}^{\circ }$ C using a Brillouin optical time-domain analyzer (BOTDA) for the first time, to the best of our knowledge. Two jacketed fibers and small-diameter carbon/polyimide-coating single-mode fiber were used for the purpose of investigating the jacket effect of thermal-strain detection on the rail. Although a nonlinear response to the temperature of the loose jacketed fiber was found, it was applicable for thermal strain monitoring when glued on the surface of the rail sample. The measured thermal strain in the rail specimen was validated by the results obtained by the strain gauge. The thermally induced strain from the large rail specimen was found to have suppressed the nonlinear impact of the fiber jacket.