In this paper, an innovative method as a coupled numerical model is developed for assessing the interaction between wagon dynamics and transient fluid slosh. This model can be considered as a robust and effective computational tool for investigation of sloshing in tank vehicles in different tank conditions and manoeuvres. Fourth-order Runge-Kutta method is adopted for solving the 19 degrees of freedom (DOFs) wagon dynamics model. The three-dimensional model is adopted, which includes car body, bogies and wheel-axles with longitudinal, vertical, pitch, and roll vibrations. Transient fluid slosh is analyzed using the computational fluid dynamics method (CFD) based on the Navier–Stokes equations combined with the volume of fluid technique (VOF). Also, the sloshing test setup is developed for validation of the multi-phase CFD simulation. By coupling of the wagon dynamics model with the fluid slosh model, the dynamic response characteristics of the railway tanker are analyzed under straight-line braking manoeuvre. The numerical parametric study is conducted to investigate the effect of filling volume and viscosity.