# Research on Response Postures of Subway Train in Straight Line Collision
## 1 Introduction  
At present, urban rail transit systems in countries around the world have made significant progress, providing people with more convenient transportation options. With the increasing dependence on rail transit, the complexity of rail track alignment and train operation control also rises, leading to safety issues that cannot be ignored. Over the years, serious train collision accidents have occurred occasionally [1, 2, 3, 4, 5]. In response to these issues, scholars have conducted extensive research on train crashworthiness design and evaluation, collision response postures, and derailment mechanisms. In the area of train crashworthiness design and evaluation, researchers primarily focus on the study of energy-absorbing devices. More efficient energy absorption devices and layout schemes were proposed, and the longitudinal stiffness of the train was optimized to maximize the safety of personnel [6]. The study of collision response postures and derailment mechanisms can provide guidance for crashworthiness design to further enhance passive safety in train collisions. Due to the nonlinear behavior of materials, large deflections and rotations, multiple units, and varying collision conditions, the response postures of the train in collision are highly complex. Theoretical modeling and prototype test studies are extremely difficult,and the size effect of scaled tests can not be avoided. Numerical simulation is the most widely used and effective method for studying the collision response of urban rail trains. In numerical simulation studies of train collisions, most research simplifies the train's front and carriage to rigid bodies, or only treats the train front and vehicle ends as elastic-plastic bodies or nonlinear springs. For example, regarding the study of train front ends, [PERSON] et al. [7] investigated the impact of the shape (circular, square, and hexagonal) and configuration of the crash modules on energy absorption for medium-speed trains. They constructed a finite element model of the medium-speed train to simulate a collision, treating the train front shell as a deformable body and the train structure as a rigid body. [PERSON] et al. [8] simplified both the train front and carriages as rigid bodies and used multibody kinematics to analyze the dynamic response of the train during a collision. [PERSON] et al. [9] simplified the middle part of carriages as a mass point and performed collision simulation of the local structure at the front and carriage ends, obtaining the force-deformation characteristics of the main energy-absorbing parts of the train. They then simplified the energy-absorbing parts into nonlinear springs in a train set collision simulation. [PERSON] et al. [10] also simplified the carriage as a rigid body and analyzed the crashworthiness of coupled trains based on machine learning methods. To further analyze the failure mechanisms of coupled trains, they set the train front as an elastoplastic body [11], while the carriage remained a rigid body, and analyzed the destruction process of the train front frame during a high-speed train collision with a rigid wall. These simplification approaches are beneficial for improving computational efficiency, but the neglect of full or partial deformation of the carriages hinders the accurate capture of the true energy-absorbing characteristics and response postures of urban rail trains under various and complex collision conditions. Some studies have also conducted numerical simulation analyses of collisions using elastoplastic urban rail train models. When conducting a rigorous assessment of a train's crashworthiness based on existing standards, it is necessary to use an elastoplastic finite element model for train collision simulation [12,13]. [PERSON] et al. [14] modeled both the train front and carriages as elastoplastic bodies and analyzed the failure process of coupled train collisions based on a subway collision accident in Beijing. They found weak points in the train front frame that need to be considered in collision analysis.