To reasonably evaluate the post-earthquake traffic function of railway bridges, a method based on the fault tree model was proposed for assessing the functional state of railway simply supported bridges. First, a fault tree model was developed with the post-earthquake functional state as the top event. Then, seven kinds of 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 with different seismic failure modes. Finally, the proposed method was applied to analyze typical railway simply supported bridges with the span length of 32 m in southwest China. The results indicate that the primary seismic failure modes of bridges involve combined damage to bearing and other components. With the increase of earthquake level, the post-earthquake traffic function of railway bridge decreases significantly. The failure mode involving combined damage of the rail and other components is the key factor contributing to the decline of the post-earthquake traffic function of railway bridges.