# Influence of Posture Change on Train Running Safety under Crosswind
## 1 Introduction  
Trains are becoming a more significant mode of high-speed and cost-effective public transportation for passengers and freight. When a crosswind exists, the running train is subjected to a speed limit [1], which has a significant effect on carrying efficiency. Operational protection is also threatened by the crosswind [2]. In fact, many derailment accidents due to crosswinds have been reported around the world, such as in China [3, 4], Japan [5, 6], and the UK [7, 8]. Furthermore, the vehicle's violent vibration do not only affect safety but also passenger comfort [9], and can deteriorate vehicle-infrastructure interaction performance [10]. As a result, research into train aerodynamic performance and running safety in crosswinds has become popular [11, 12, 13]. In response to the train overturning accidents mentioned above, a large number of studies have been carried out through full-scale tests [14], scaled model tests [15, 16, 17], and numerical simulations [18, 19] to study aerodynamic performance, the surrounding flow field distribution, the operational safety of static or dynamic trains, and the effect of wind protection facilities in crosswind regions. Among these, numerical simulation methods have received more attention in recent years [20, 21] due to their reliable results, low cost, and easy control of test variables. Static trains are used in the majority of numerical investigations. Although such research approaches have a cheap computational cost and are versatile in terms of computing the flow field around the train, they are unable to replicate the train's movement when the transient flow field and the crosswind are coupled. As a result, several investigations on trains in crosswinds have been conducted with a dynamic method. The wind-train-bridge coupling is studied using a dynamic analysis model. In the research conducted by [PERSON] et. al. [11], the effects of a wind barrier with a height of 3 m and a porosity of 30% on the aerodynamic coefficient, flow fieldstructure, and running protection of high-speed trains under crosswind are explored in the bridge-tunnel section; results showed that the wind barrier reduced the sharp change effect of the aerodynamic coefficient by more than 50%. [PERSON] et. al. [22] conducted an analysis of the wind-train-bridge dynamic coupled system and the dynamic responses of the train in the tunnel-bridge-tunnel infrastructure in the canyon wind environment were explored. [PERSON] et al. [23] discovered that trains passing through a tunnel in a crosswind experienced transient changes in the flow field structure and aerodynamic characteristics, which reduced the running safety of the train. It has been proven that the yaw angle has a significant effect on the aerodynamic forces and moments of the train. However, the train is subjected to unbalanced and unstable forces during operation, and its posture is bound to change during the entire operation. Changes in some degrees of freedom, including the yaw attitude that can directly determine the yaw angle, will change the flow field around the train and its aerodynamic performance, thereby affecting the safety of the train's operation [24]. According to a previous study [25], in terms of vehicle dynamics and as a result of relative lateral displacements in the wheel-rail contact site, primary and secondary suspension between tracks, wheelsets, bogie frames, and car bodies, the gravitational center of bodies will deviate from the central line of track. It was also proven that when a train runs on a bridge suffering a crosswind, its attitude varied, as did the flow field, aerodynamic loads, and running safety indices. However, in the studies of different train postures, only the running on a bridge is considered, which is a relatively specific operating environment. However, a train tends to run more on flat ground, and its aerodynamic response performance and running safety will be different form that on the bridge.