## 1 Introduction  
The flow field around a vehicle usually varies transiently when vehicles move from one infrastructure scenario to another, depending on the infrastructure scenarios, such as flat grounds [1; 2; 3; 4], embankments [1; 5; 6], and viaducts [7; 8; 9], thereby resulting in the transient variation of aerodynamic force on a vehicle and even a serious overturn accident. For example, an overturn accident occurred in China's Lanzhou-Xinjiang Railway in 2007, which might be attributed to the transient variation of aerodynamic force when the train was entering wind barriers from open air [10; 11; 12; 13; 14]. The transient variation effect is likely to be serious when vehicles are driven into the tunnel from a crosswind environment because the tunnel has more considerable space closure than other infrastructure scenarios. In addition, entering/exiting a tunnel is becoming a common operation environment of a high-speed trains (HSTs), especially in China where an increasing number of cut-and-cover tunnels are designed as a windshield facility of a windy area (Figure 1).  
[PERSON] et al. [12] investigated the temporal evolution of the flow structure and conducted aerodynamic behavior using a numerical simulation method to study the transient characteristics and main factors of the aerodynamic loads that act on trains during tunnel entry with crosswind. The results showed that the transient variation of aerodynamic forces on trains was significant and varied differently for the leading, middle, and tail carriages. Although only a quantitative analysis, the train movement presents a complex dynamic response, including the lateral swing, the snake-like, and pitching movements when a HST enters a tunnel in a crosswind environment. The transient variation of the aerodynamic load is usually a crucial source of a vehicle's safety risk associated with crosswind [7; 15; 16; 17; 18]. The flow characteristics around a HST when exiting from the tunnel under crosswind has been researched by [PERSON] [19]. The results show that the maximum yawing and rolling moments may occur when about one third of the train body is out of the tunnel. To date, although considerable attention has been given on various dynamic responses of vehicle aerodynamics, limited attention has been devoted to focusing on the transient aerodynamic response and the corresponding traffic safety of a running train induced by wind effect in a tunnel entrance [15; 16; 20; 21; 22; 23; 24].  
As a follow-up work of a previous study [12; 25], the present work focuses on the transient aerodynamic response and the corresponding traffic safety of a running HST during tunnel entry with crosswind. Therefore, a full multi-body system (MBS) with 31 independent degrees of freedom (DOFs) of the vehicle is adopted to discuss the wheel-track dynamic response induced by the sudden extinction of wind effect on the train at the tunnel entrance. Thus, safety coefficients, such as the wheel load reduction rate (WRR) and derailment coefficient (DC) are obtained. Finally, the characteristic wind curve (CWC), which shows the wind velocity against the train speed, is also presented for application based on the safety criteria.  
This paper is organized into six sections. Section 2 presents the wind-train-track coupling dynamic system. Section 3 proposes the approach applied to simulate the transient flow field and calculate the aerodynamic loads when HSTs enter a tunnel under crosswind. Section 4 provides the numerical demonstration and the verification. Section 5 presents the temporal evolution of aerodynamic loads and safety coefficients. Finally, Section 6 presents some conclusions and some suggestions for future work.