When a vehicle is running, wheel is generating vertical and lateral force on the rail, in addition to load of vehicle, through a complicated set of motions. The derailment coefficient refers to the ratio of lateral force to vertical force(wheel load), and if the value exceeds a certain level, a wheel climbs or jumps over the rail. That's why the value is used as a criterion for running safety. Derailment coefficient of rolling stocks alters according to shape of rail track. I measured threedimensional angular velocity and acceleration to use 3D Motion Tracker. Test result, derailment coefficient of rolling stocks and shape of rail track examined closely that have fixed relation. Specially, was proved that roll motion has the close coupling relation.
