# Research on Response Postures of Subway Train in Straight Line Collision
## 6 Response Postures Analysis
### Analysis of the Response Processes and Causes of Different Response Postures
#### 6.2.3 Lateral Buckling  
The lateral buckling is most obvious in the front-side collision with a rigid wall. Taking the impact speed of 36 km/h as an example, the displacement time history curve (vertical and lateral) of the front middle position of the first carriage and the lateral contact force time history curve between Wheelset-1 (i.e., the first Wheelset) and the track are extracted under each working condition, as shown in Figures 24-26. Figure 23: Lateral and vertical impact force on wheelset. Figure 22: Derailment and large lateral movement of the train front in oblique collision. Based on the simulation results, under the front-side impact conditions of C1, C2, and C3, the front wheelset of the first carriage left the track at approximately 0.1 s. The displacement history curve illustrates the amplitude of the pitching and yawing motions of the first carriage across the four conditions. In the frontal impact condition C0, prior to 0.1 s, there is almost no noticeable head shaking. However, under the other conditions, the first car experiences varying degrees of lateral and vertical displacement. Under the combined effects of nodding and shaking motions, the front end of the carriage exhibits a downward-slanting movement trend. The impact force is transmitted from the car body to the axle through the suspension system, as shown in Figure 26, where the wheel experiences a lateral force from the track. As a result, Wheelset-1 at the front of the first carriage jumps off the track. When the lateral movement of the obstacles is consistent, but the collision speed differs, as depicted in Figure 27, it can be observed that before the wheelset derails, an increase in speed leads to a rise in both the amplitude and frequency of the nodding motion. Consequently, the risk of train derailment increases with higher speeds. Figure 26: Lateral impact force on the first wheelset. Figure 24: Lateral displacement history curves of leading car. Figure 25: Vertical displacement history curves of leading car. Once the wheelset derails, it loses track constraint. As shown in Figure 28, the front of the carriage becomes prone to rotating around the Y-axis, which is perpendicular to the rail surface at the impact point. Consequently, C-1 and the rigid wall cannot remain perpendicular. As the rear carriage continues to impact, the front end of C-1 rotates around the Y-axis at an increasing angle, while the rear end of C-1 moves laterally, and the front end of C-2 also shifts laterally as the derailment progresses. As illustrated in Figure 29, the lateral displacement time curve for each wheelset shows the largest movement occurs during the collision between the first and second carriages, followed by the collision position between the second and third carriages. The displacement of Wheelset-1 exhibits the greatest transverse displacement in the opposite direction. When the train impact speed is less than or equal to 36 km/h, the greater the impact speed, the larger the lateral displacement at the bending point. [PERSON] [41] investigated the  
Figure 28: Lateral buckling under front side collision. Figure 27: Vertical displacement history curves of leading car at different impact velocity. (**a**) Vertical displacement history curves of leading car under C1 condition. (**b**) Vertical displacement history curves of leading car under C3 condition. Figure 29: Lateral displacement history curves of wheelset under C1 condition. main causes of train lateral buckling behavior through dynamic modeling analysis and found that the initial yawing motion of the vehicle connection structure has a direct impact on the lateral motion of the vehicle during the collision process. When train wheelsets run along the track, their lateral degree of freedom generates a periodic lateral oscillation known as \"snake motion\". As the operating speed increases, the lateral stability decreases [42], leading to an increase in the amplitude of lateral oscillation, which in turn increases the yawing motion of the vehicle.