Natural frequencies of engineering structures often vary with environmental temperature changes. For bridges, these variations can affect structural performance and potentially compromise the safety of pedestrian and vehicular traffic. The extent of this impact depends on bridge type and boundary conditions. One simply supported steel–concrete composite railway bridge in northern Sweden, where temperatures range from –40 °C to 30 °C, is investigated in this study. Using finite element analysis (FEA), the effects of temperature-induced changes in concrete properties, boundary constraints, and ballast stiffness on the bridge’s natural frequencies and modes are examined. Results show that: (1) As temperature decreases, the first, second and third natural frequencies increase by 12.41%, 20.08% and 16.92%, respectively; (2) the frequency–temperature relationship exhibits a trilinear behavior; and (3) temperature variations have a more pronounced effect on torsional modes than on bending modes. This study attempts to provide advice for the scheme and maintenance of railway bridges in cold regions.