Today, there are a large number of masonry arch bridges in service within railway networks. These old infrastructures have been designed for service loads in the past 100 years. Owing to increased service loads, it is essential to investigate the behavior of masonry arch bridges. Several empirical, analytical and numerical studies have been conducted to estimate the residual capacities of masonry arch bridges. The present study aims to employ the extended finite element method (XFEM) to obtain the ultimate capacities of old arch bridges for the first time. By adopting a non-geometric approach in fracture mechanics and defining initial structural defects, XFEM can easily predict the failure and ultimate capacity of a structure by using the crack growth pattern. Thus, the present study made use of two ageing railway arch bridges that are known to be cracked structures to build a three-dimensional XFEM model. The ultimate capacities and safety factors of the bridges are studied by defining several initial defects in different conditions. Finally, using experimental outcomes, the nonlinear finite element method along with the concrete damaged plasticity model are exploited to validate the results. The results indicate that XFEM agrees very well with the other methods. Considering its unique flexibilities, XFEM can be employed effectively and efficiently to analyze ageing cracked masonry arch bridges.