External tendons in high-speed railway bridges are expected to last 100 years. However, failures have been reported after only 25 years, mainly due to grout disaggregation, steel strand corrosion, and fatigue from stress cycles. To address this issue, a methodology is proposed to assess the structural behaviour of external post-tensioning tendons throughout their life cycle, considering stress-corrosion fatigue effects. This approach integrates two existing models: (i) a corrosion model and (ii) a fatigue assessment model. The corrosion model leads to section loss through uniform and pitting mechanisms, increasing strand stress and potentially causing failure. The fatigue assessment model, based on cumulative damage methods, evaluates tendon fatigue yearly using the rainflow counting method. The S-N curve is modified to account for stress levels and corrosion effects. A railway bridge with six external post-tensioning tendons is used as a benchmark structure. A numerical modal analysis provides modal properties to develop a state-space model predicting the bridge’s dynamic response under moving train loads. Tendon strain and stress are assessed at midspan and deviators. Given the many influencing variables, a sensitivity analysis is conducted considering fuzzy variables. This study enhances the prediction of tendon lifespan under varying conditions, contributing to improve durability assessment tools for external tendons in railway bridges.