## 1 Introduction  
Currently, railway vehicles need to run on or through long-span bridges, i.e., Hutong Yangtze river railway bridge, Jinshajiang railway bridge, and Strait of PingTan Bridge [1]. The serviceability of trains running on such long-span railway bridges is of major concern around the world [2, 3, 4]. In order to predict, control, and improve the serviceability of the train-bridge system in crosswinds, their aerodynamic properties and underlying flow physics should be fully investigated in advance. From the train point of view, modern vehicles are more sensitive to crosswinds due to their higher operating speed and lighter weight [2, 5]. To guarantee the operating comfortability and safety of a train running on scenarios of different types (flat ground, embankment, viaduct, and long-span bridge) in crosswinds, two kinds of risk analysis methods have been put forward, i.e., the mathematically rigorous approach developed by many previous researchers [6, 7, 8, 9, 10, 4] and the characteristic wind curve (CSC) as documented in specification [11]. However, no matter which method is utilized, the wind loads acting on the train are the basic input parameters. From the infrastructure scenario point of view, the long-span railway bridge is generally the most critical site in a segment [12]. Relative to other scenarios, the long-span railway bridge is characterized by smaller stiffness, lower damping, lighter weight, and severer wind environment. Moreover, the dynamic responses of the train and bridge system induced by crosswinds are chiefly resisted by the bridge. Given all the above characteristics, the crosswind aerodynamic properties of the railway bridge scenario should be taken into consideration in the risk analysis of the train and bridge system. Within the range of the CSC, the bridge deck may suffer from buffeting and vortex-induced vibration [13; 14; 15], which should have a strong negative effect on the serviceability of the train in crosswinds. To assess the dynamic responses of the long-span railway bridge induced by crosswinds, the aerodynamic properties of the bridge deck need to be measured, analyzed, and classified. From the train-bridge system point of view, the aerodynamic and dynamic interactions between the train and bridge are very complex [16; 17; 18]. On one hand, the presence of the bridge deck should significantly impact the approaching and underbody flows of the train. For example, the leading-edge separated flow of the bridge-deck may lead to an overspeed effect on the approaching flow of the train, similar to [PERSON]'s hypothesis [19]. Meanwhile, the flow separation-reattachment of the bridge-deck would be noticeably altered by the presence of the train. On the other hand, the dynamic responses of the train-bridge system are obviously intensified in crosswinds [4; 6; 7; 8; 9; 10]. The above aerodynamic and dynamic interactions were mainly carried out on train-viaduct and train-truss bridge systems [4; 18; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29]. Associated studies on the train and flat closed-box bridge system are limited. Currently, the flat closed-box deck has been widely utilized in long-span railway bridges in China since 2014. For such a wind-sensitive bridge deck, systematic investigations on the aerodynamics of the train and flat closed-box bridge system are urgently required. Generally, the relative spatial position of two objects (i.e., circular cylinders, square and rectangular prisms) plays a determining role in their aerodynamic forces, flow structures, pressure distributions, velocity fields, and near wakes [30; 31; 32]. When it comes to the train-infrastructure system, the change of the relative spatial position of the train and infrastructure is mainly influenced by the variations of the occupied track the train is running on. In the past decades, several studies have already been carried out to discuss the influences of the aforementioned occupied track variations on the aerodynamic properties of trains [12; 22; 26; 33].