## 3 Multibody Simulations
### Dynamic Responses of Vehicle When Accelerating through Curved Tracks  
When the wind speed decreases (below 15 m/s) and the vehicle speed is below 150 km/h, the train should be re-accelerated to return to operating speed. Therefore, to study the influence of the IVS on the dynamic characteristics of the train re-accelerating through curved tracks, the initial working conditions of initial velocity \(V_{0}\) = 150 km/h, \(R\)7000 m, and \(V_{w}\) = 15 m/s were set.  
As can be seen from Figure 16a, the unloading factors of the four bogies FF, FB, MF, and BF are increased when the IVS are taken into account under acceleration conditions. Figure 16b shows that considering the IVS increases the maximum derailment coefficient for all bogies except the FF bogie. Of the six bogies, the FF bogie has the highest unloading factor of all bogies. Considering the IVS, the maximum values of the unloading factor for FF, FB, MF, and BF bogies were increased by 0.11%, 1.05%, 0.36%, and 7.27%, respectively. Similarly, the FF bogie has the maximum value of derailment coefficient. Considering the IVS, the maximum values of the derailment coefficient for FB, MF, MB, BF, and BB bogies were increased. Particularly, the BF bogie was increased by 6.94%. Since only the head car and tail car are motor cars during acceleration, the longitudinal acceleration of the car bodies will be different. The motion and impact effects between the vehicles will be more realistic if the suspension between the vehicles is considered rather than a simple fixed connection. Therefore, considering IVS will slightly increase the unloading factor and derailment coefficient. The safe speed of the vehicle will be reduced, and the results will be closer to the real situation.  
It can be seen from Figure 17a that considering the IVS, the peak of the lateral vibration acceleration of the head car when driving out of the spiral transition curve track is reduced by 55.13%. Considering the IVS, the peak of the lateral vibration acceleration of the middle car is reduced at all four transitions in the curve track. However, as shown in Figure 17c, the peak of the lateral vibration acceleration of the tail car when entering the spiral transition curve track is decreased by 68.17%. For the same reason, only the head car and tail car are motor cars during acceleration, and the longitudinal accelerations of the head car and tail car are different from the middle and cause more impact between the vehicles. In the case of fixed connections, the motion between the bodies is ignored. Hence, the lateral acceleration of the vehicle bodies will be larger but more realistic when considering the IVS.  
Figure 16: Dynamic responses of the vehicle when accelerating through curved tracks in two cases. (**a**) Unloading factor. (**b**) Derailment coefficient.  
As shown in Figure 18, the body roll angle is the largest of the body motion angles during train acceleration in crosswind conditions. The IVS has little effect on the maximum roll angle of the head car F and middle car M. However, it increased the maximum roll angle of the tail car by 6.05%. Considering the IVS has a great influence on the maximum value of pitch angle of the three car bodies. The pitch angle of the head car, the middle car, and the tail car increased by 39.71%, 8.6%, and 130.89%, respectively. This is due to the existence of backlash in the coupler buffer device, but the fixed connection does not allow for relative displacement between the car bodies. For the yaw angle of the car body, considering the IVS will reduce the yaw angle of car F and car M by 4.70% and 37.59%, which is also due to the fact that dampers between the vehicles can suppress the yaw motion between the car bodies.