## 1 Introduction  
The China Railway Track System Type II (CRTS-II) ballastless slab track is widely used in China's national high-speed railway system, due to its various advantages, such as running comfort and high structural stability [1; 2; 3]. As the ballastless track-bridge structural system composed of CRTS-II slab track is a vertical multilayer spatial structure, the local temperature and overall temperature are significant factors that inevitably impact the mechanical properties of the structural system and affect the running comfort and safety of the high-speed trains, increasing the difficulty of maintenance. Thus, the influence of the temperature on the mechanical performance of the track structure cannot be ignored [4; 5]. Recently, scholars have carried out theoretical and experimental studies on the temperature distribution of bridge structures [6; 7; 8; 9; 10]; analyzed a series of influencing factors on the temperature and stress distributions in concrete box girder bridges; and presented analytical models to predict temperature and stress distributions. The temperature distributions in concrete box-girder bridges were analyzed [11; 12; 13; 14; 15] and empirical formulas were proposed to predict the maximum vertical and lateral temperature gradients. As one of the most significant factors producing interfacial cracks [16] and longitudinal buckling [17] in ballastless tracks, the temperature distribution has a great influence on the thermal behavior of ballastless tracks [18; 19; 20; 21; 22]. The main causes of the structural diseases in ballastless track structures are the variation of the ambient temperature and the difference in thermal behavior between the cement emulsified asphalt mortar (the CA mortar) and the concrete [23; 24; 25]. In recent years, research on the temperature distribution in CRTS-II ballastless tracks has increased. However, most existing research has focused on track slabs with subgrades [26; 27; 28]. The few studies on the temperature distribution in CRTS-II ballastless tracks on high-speed railway (HSR) bridges mainly focused on the theoretical solution under periodic climate conditions [29]. In addition, the existing research methods on temperature distribution analysis mainly included theoretical analysis methods based on finite element calculations and statistical analysis methods based on monitoring tests. [PERSON] et al. [30] established a thermodynamic analysis model of CRTS-II ballastless track based on finite element software and studied the vertical temperature distribution of the ballastless tracks under different climatic conditions, but due to the lack of accurate thermodynamic parameters of the CA mortar in the model, it was difficult to form a comprehensive understanding of the overall temperature distribution in the track. [PERSON] et al. [29] fitted the average vertical temperature difference distribution in the track by exponential function and obtained the daily changing law of the transverse temperature distribution in the ultra-high section of the track based on statistical analysis of the daily temperature data monitored on a CRTS-II ballastless track. However, due to the lack of temperature gauges in the CA mortar, the transverse and vertical temperature distribution in the track cannot be completely described. In summary, there is still no heating test method to control the temperature rise and drop in ballastless tracks. Little attention has been paid to the CA mortar layer, and research on the transverse and vertical temperature distribution under rapid, extreme high temperature rising and dropping conditions is scarce. Meanwhile, due to global warming, the extreme high temperatures are becoming more frequent, and rapid temperature rising and dropping is inevitable.