To enhance the dynamic performance and structural safety of C70E gondola cars, a coupled analytical method integrating finite element simulation with multi-boundary modal testing was employed to systematically investigate vibration characteristics and modeling optimization mechanisms in railway freight vehicles. Experimental procedures were conducted using advanced computational techniques. Results demonstrate that the weld-line connection model significantly reduces modal errors compared to traditional surface connections, with transverse first-order reverse bending frequency deviations optimized from 46.78% to 0.91%. Dynamic equivalence techniques for boundary constraints were shown to restrict maximum relative errors within 7.48% for track supports, 8.47% for bolster supports, and 7.8% for wheelset bearing beam supports. An iterative correction strategy based on the Lanczos method was verified to improve transverse high-order bending modal accuracy by an average of 18.6%. The study confirms that a dual-coupled modeling system integrating welds and boundary conditions effectively resolves stiffness distortion issues inherent in conventional simplified models. The developed modal analysis methodology is demonstrated to provide critical technical support for structural optimization and safety evaluation of heavy-haul freight vehicles.