In order to evaluate the structural fatigue reliability of the 70 t railway flatcar NX70A carbody during long-term service, a comparative analysis between the AAR-1001 standard equivalent load spectrum and the measured load spectrum from existing lines was conducted, the corrosion conditions of the main structural beams were investigated and statistically analyzed, and the fatigue damage per unit mileage, safe service life, and the contribution of different load types to cumulative damage at critical points were evaluated based on the AAR standard fatigue life calculation method for welded structures, considering various load spectra and corrosion states, to investigate the fatigue damage characteristics of the NX70A flatcar carbody.The results show that the average annual corrosion depth of the primary beams ranges from 0.037 mm to 0.055 mm, with more severe corrosion observed at upper positions such as the top flange and upper cover plate; vertical loads contribute predominantly to the fatigue damage at most fatigue-critical points, whereas the contribution of longitudinal loads to damage at most key locations is less than 10%; and fatigue reliability assessment of freight car bodies based on the AAR standard equivalent load spectrum is insufficiently conservative.It is concluded that an accurate evaluation of the fatigue life of this type of freight car carbody must account for the effects of actual in-service load spectra and structural corrosion conditions.