A new approach for simulating the liquid sloshing in a tank car is proposed based on the flexible multibody dynamics (MBS) and the absolute nodal coordinate formulation (ANCF) in describing the large deformation. It is used to study the non?linear free?surface of the liquid during sloshing,and it can be applied also to study the liquid sloshing problem within a container with the complex geometry. Based on the Newton fluid,derivations of the fluid constitutive equations are provided as well as the incompressibility characteristics. The ANCF solid element is applied to mesh the liquid inside the tank car,the penalty method is used to develop the fluidtank contact model,and the fluid?tank coupled MBS model is then built. Furthermore,numerical simulations are conducted to demonstrate the liquid sloshing inside a railway tank,in which harmonics are applied along the lateral and longitudinal axes,respectively. It shows that the free?surface has a non?linear shape and height,which experiences different values for different tank cross?sections.