# 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  
#### 6.2.1 Climbing Between Carriages and Vertical Arch  
Within the range of impact velocities studied in this paper, the failure process between the carriages can be summarized as follows:  
1. End Deformation: After the first impact, due to the deceleration of C-1 and the collapse of coupler buffer device between the carriages, a direct impact occurs between the end of C-1 (which has slowed down) and the higher-speed C-2. The collision ends undergo large plastic deformation.
2. Climbing: After decelerating, C-2 collides with C-3, leading to a second impact between C-1 and C-2. The plastic deformation at the collision end of C-2 increases, and its ability to continue absorbing the impact diminishes. Due to the inconsistent pitching and yawing motions of the carriage, the relative distance between the ends of the carriages increases both vertically and horizontally. The rear end of the bottom plate of C-1 rides over the front end of the bottom plate of C-2 (as shown in Figure 21), and Wheelset-2 at the rear of car C-1 begins to lift off the rail surface.
3. Vertical Arching: As the impact from the rear carriages continues, the overlap between C-1 and C-2 increases, and the end structure is severely damaged (as shown in Figure 21). Eventually, the two ends begin to arch vertically while remaining embedded in each other, leading to a significant increase in the relative vertical displacement between the wheel and the rail. If the impact energy is high enough, this damage process will occur similarly between the rear carriages.
4. Overturning/Lateral Buckling: After the train speed drops to zero, the cars that were vertically arched fall back, while those that have shifted laterally or are angled relative to the track are unable to return to the track surface. This results in lateral buckling of the train and eventual overturning of the carriages.  
When the train impacts a rigid wall at the front or front-side, the maximum relative vertical displacement (i.e., the vertical arch amplitude) between the wheel and rail increases with speed. When the speed exceeds 36 km/h, the height of the wheelset lifting off the ground due to this response exceeds 1 m. This results in significant impact and overturning as the carriages fall back.  
Figure 21: Overriding phenomenon and vertical arch.  
In the simulation, the addition of an anti-climbing device between C-1 and C-2 significantly reduced end damage and the climbing phenomenon, with the vertical arch amplitude dropping to 0.1 m. Therefore, the primary causes of the climbing phenomenon and the subsequent severe vertical arching are the serious plastic deformation at the carriages' end and the asynchronous pitching and yawing behavior between adjacent carriages. [PERSON] et al. [17] used a combined multi-body dynamics and finite element analysis to study car-climbing behavior during train collision, also noting the impact of asynchronous pitching and height differences at the carriages' ends on the climbing behavior.