Due to the limited terrain conditions, an increasing number of railway lines have to be designed with long inclines, rendering traditional wheel-rail force measurement technology, which only addresses lateral and vertical wheel-rail forces, inadequate for addressing the subsequent operational and maintenance issues. Further research into longitudinal wheel-rail force measurement technology is required to effectively address these challenges. Therefore, this paper introduces a longitudinal wheel-rail force measurement method based on the LM algorithm. Initially, the measurability of longitudinal force is validated and its governing principles are investigated through finite element simulation calculations. Subsequently, a longitudinal force measurement bridge is designed as a foundation for transforming the problem of solving longitudinal force into that of solving an overdetermined multivariate nonlinear equation system. Then, the degree decomposition is used for fitting calibration coefficient under arbitrary angle and the rotation angle and wheel-rail force is solved through the LM algorithm. By employing SIMPACK to simulate train uphill operation under diverse conditions, this study validated the proposed method and quantified the influence of contact point on longitudinal force measurement error. The results demonstrate that the proposed method exhibits high precision, with a relative error of less than 6%.