Abstract: In establishing the dynamics equation for the simplified wheelset model of the train, it is assumed that the lateral displacement and yaw angle are small and the height of the center of mass remains constant. The approximate expression of the equivalent gravity stiffness and rolling radius can be obtained easily. In this paper, the functional relationships between the height of the centroid of the wheelset and the rolling radius with the lateral displacement are given by geometric method, so that the analytical expressions of these quantities can be obtained. In addition, the simple calculation methods of the equivalent gravity stiffness and rolling radius are proved to be correct by kinematic method.