A traditional rail-beam-pier plane model and a rail-beam-pier grillage model were built by adopting the mechanism of ideal elastic-plastic ballast resistance with the background of a (36+112+36) m double-track continuous beam-arch composite bridge on the new Chengdu-Lanzhou Railway line. The aim was to study the difference between grillage models and plane models about the distribution of longitudinal forces in continuous beam-arch composite bridges. The results show that the distribution of longitudinal forces is basically the same in the grillage models and plane models, but the results of the plane models are conservative. Plane models can not accurately reflect the spatial mechanical properties with multi-track under asymmetric loads. Therefore, the grillage model is more effective in the calculation of CWR(continuously welded rail) on the multi-track long span railway bridge.