The condition of box girders in high-speed railways directly determines track smoothness and operational safety.Slow and minute variations in girder displacement pose challenges for monitoring,particularly in overcoming strong electromagnetic interference generated by electric traction systems and achieving automated monitoring.A key difficulty lies in the varying initial widths of expansion joints among different box girders,necessitating equal-precision measurement for comparative analysis of their changes.This study proposed an optical low-coherence expansion joint displacement sensor for high-speed railway box girders,which was subjected to vibration resistance testing under laboratory conditions.The sensor's performance under common low-frequency vibration scenarios was verified,followed by its application in monitoring the expansion joints of simply-supported box girders on the Beijing-Tianjin Intercity High-speed Railway.Results show that,influenced by sunlight and temperature,the difference in displacement between the top and bottom surfaces of expansion joints can reach 150 μm,with the sensor demonstrating a long-term monitoring accuracy of 50 μm.These findings provide valuable reference for the implementation of automated monitoring of expansion joints in high-speed railway box girders.