In order to reduce post-derailment consequences, the motion and dynamic performance of derailed vehicles were investigated based on field derailment test. A high-speed railway vehicle was used to be conducted a field derailment test. Test results indicate that the motion of derailed vehicles includes impacting, collision, bouncing and rolling on the track after falling out of rails. At the initial of derailment, fasteners were severely destroyed by derailed vehicle because lateral distance between rails and derailed wheels is smaller than lateral distance between rails and fasteners. In addition, dynamic responses of axle box appears continuously equally interval impact vibration because of fasteners. Gearboxes or motors can catch rails after a derailment which can limit lateral deviation and keep derailed vehicles close to the track centerline. Thus, two principles are proposed to optimize structure of gearboxes and motors in order to minimize losses of derailment accidents.