Temperature-induced expansion and contraction of the upper highway steel girder can modify the force distribution in the vertical hanger cables and thereby influence the response of the lower railway deck in highway–railway steel composite bridges. This study analyzes three years (2019–2021) of field monitoring data to quantify the relationships among member temperature, highway expansion-joint displacement, and inner/outer cable tensions. Linear temperature-based prediction equations were developed using daily-averaged records and validated against independently estimated cable tensions from vibration-based identification (n = 24 tests; 8 cables × 3 campaigns). The prediction showed mean deviations below 5% and a maximum absolute deviation of 8.4%. A supporting ANSYS model reproduced the first-mode frequencies within 4%. The proposed framework provides practical equations for operational monitoring and maintenance planning within the monitored temperature range.