In order to reasonably evaluate the post-earthquake traffic function of railway bridges, a methodology founded upon a fault tree model was proposed for the purpose of assessing the functional state of railway simply supported bridges. First, an investigation was conducted into the correlation between damage to bridge components and post-earthquake functionality of the structures. To this end, a fault tree model was developed, with the post-earthquake functionality of the bridge designated as the top event. Thereafter, seven seismic failure modes were defined according to the basic events of the fault tree. A post-earthquake functional state assessment process for bridges was established considering different seismic failure modes. Finally, taking typical railway simply supported bridges with a span length of 32 m in Southwest China as a reference, this study used OpenSees to establish 1 000 full-bridge finite element models with different structural parameters. The proposed method was applied to assess the seismic failure modes and post-earthquake functional states. The findings indicate that the primary seismic failure modes of bridges involve combined damage to bearings and other components. With the increase in earthquake level, the post-earthquake traffic function of railway bridges decreases significantly. The failure mode involving combined damage to the rails and other components is the key factor contributing to the decline in the post-earthquake traffic function of railway bridges.