## 1 Introduction  
Slipstreams are the flow induced by a moving train, which can cause damage to the infrastructure and destabilize the people near the railway line [1; 2]. In recent years, the train-induced slipstream has become a serious safety concern with the spectacular expansion of railway networks and increasing train speeds [3; 4]. For example, 16 incidents have been reported in the UK in 32 years since 1972 [5]. More recently, a number of accidents caused by slipstreams were also reported in Switzerland and Austria (two infant fatalities) [6], emphasizing the importance of understanding the slipstream and requirements for a systematic investigation to avoid the induced issues. On the other hand, high-speed trains (HSTs) running on a bridge have become a common scene due to the high bridge ratio in the high-speed railway network. The bridge ratio is usually larger than 50% in the high-speed railway network in China. Particularly, the maximum bridge ratio reaches 94.2% on the Guang-Zhu intercity railway [7]. To study the train-induced slipstream, there are four main methods: full-scale measurement [8; 9; 10], the moving model test [11; 12; 13], the wind tunnel test [14; 15] and numerical simulation [16; 17]. Full-scale measurement is an effective way to obtain data from the slipstream of HSTs. Using a full-scale test, [PERSON] et al. [8; 9] measured the slipstreams for several different types of passenger trains in Europe to examine their differences and commonalities. Following the experiments in [8; 9], [PERSON] et al. [18] branched out extensive research into a freight train, and the existing network trains' slipstream characteristics were summarized and analyzed. However, a full-scale field test requires a lot of environmental, safety, and cost concerns [19], and the test intervenes too late in the development process of HST-related issues [20]. In recent years, wind tunnel tests and computational simulation are becoming the two approaches widely used to investigate the features of slipstreams [21; 22; 23]. [PERSON] et al. [24] investigated the slipstream of high-speed trains in a wind tunnel, and the familiar slipstream characteristics and transient nature of the wake were analyzed. [PERSON] et al. [25] compared the train-induced flow structure and aerodynamic loads for three different ground/wheel configurations by using numerical simulations. Moreover, several other researchers [26; 27; 28] have also successfully investigated the slipstream of an HST using wind tunnel tests and computational fluid dynamic (CFD) simulations. Nevertheless, the relative motion between the vehicle and ground/infrastructure is not considered. Furthermore, not only the model used in computational simulations is similar to the wind tunnel tests, but also they require a large number of computing resources and experimental data to validate. In part because of the paucity of data, the effects of crosswinds on the slipstreams of trains at full scale are poorly understood. Only a few studies have examined the effects of crosswind on the slipstream due to the difficulty in simulating the train running under the crosswind and the unsteady nature of the issue. Investigating how crosswinds affect train slipstreams is challenging to achieve at full scale due to the multivariable ambient winds, as well as the number of trains in operation. The slipstream experienced by an observer may change under crosswinds present under realistic operating conditions. Physical modeling approaches offer controlled environments to conduct experiments, such as wind tunnel testing [29] and moving model testing [30], allowing data to be collected relatively quickly and cheaply compared to full-scale data. With advances in the technology of the hardware and control methods [31], currently, a moving model test is applicable in studying the HSTs' slipstream and has become more acceptable. It can simulate the relative motion between the train and the ground/infrastructures. Besides, the relationship between crosswind and the train is easier to consider.