# Brillouin Frequency Shift Sensing Technology Used in Railway Strain and Temperature Measurement
## 5 Conclusions  
In contrast to tests using weather stations or visual inspection, the test used in this study was found to be able to clearly identify various temperatures of a rail. We obtained a temperature difference of 12.1 \({}^{\circ}\)C between the temperature distribution of the measured rail and the atmospheric temperature, and there was 1.5 h delay between the two. In addition, we obtained rail irregularities ranging from \(-\)0.3 to +0.4 mm by calculating the slight strain difference of the rail in this test. We were also able to distinguish the unmovable and movable zones to derive the numbers and spacing of the sleepers. Ultimately, we calculated the position of the abnormal area and graphically presented it on the screen of the sensing system. This test is performed by a DFOS. The temperature and strain changes can be measured with the fiber Brillouin frequency shift, and then one can obtain temperature, strain and rail displacement values based on a reference track. By observing the strain, displacement, and temperature distribution of a rail across time, one can determine whether a railroad is abnormal. We reached several useful conclusions, including the following: (1) at any time, the temperature distribution and continuous change of a main rail can be measured with this test; (2) this test can accurately measure the geometric irregularities, displacement distribution, and abnormal positions of a rail; and (3) after setting the warning and action values of critical parameters, test data and warning messages can be automatically displayed with graphical charts. Using the above-tested foundation, we will continue to conduct relevant research and verification regarding track characteristic issues, and we will continue to improve system accuracy and efficiency. Future research directions include: (1) measuring the stress accumulation and release of continuous welded rails, as well as comparing the effects of temperature on this phenomenon; (2) testing the strain and temperature distribution of continuous welded rails, especially in curved sections, and verifying the displacement relationship; and (3) analyzing and comparing DFOS sensing results with actual maintenance engineering for a better system. This is the first DFOS test for the direct measurement of an operating main line in the Taiwan Railway. We successfully translated measurement results into graphical outcomes that are more convenient for maintenance and management teams to operate and judge. Additionally, we hope that the distributed optical fiber sensing technology can promote the benefits of track maintenance. Figure 12: Suspect location of displacement: Displacement distribution derived from the strain differential (\(\Delta\varepsilon\)) of the rail. The displacement values in colored places are worth noting. **Author Contributions:** W.-K.H. : analysis, validation, and main editing and reviewing; Y.-L.L. : software programming and computation; T.-T.K. : on-site measurement and data curation. All authors have read and agreed to the published version of the manuscript. **Funding:** This research received no external funding. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** Not applicable. **Acknowledgments:** This test and study were conducted using the DFOS and sensing fibers that were provided by OlitGlobal Technology (Kaohsiung, Taiwan) to measure the strain and temperature of the tested rail. We also thank the Kaohsiung Branch of Taiwan Railway Administration and the Southern Office of Railway Reconstruction Bureau for their assistance during the test period. **Conflicts of Interest:** The authors declare no conflict of interest. The sponsors had no role in the design, execution, interpretation, or writing of the study. **Nomenclature**