## 1 Introduction  
With its advantages, such as high smoothness, high stability and good durability, the CRTS-II ballastless slab track has been widely used in China [1; 2; 3]. However, under the joint action of construction, the natural environment, external loading, self-aging, and other factors, the CRTS-II ballastless track system will experience a series of issues, such as interlayer separation [4], longitudinal irregularities [5], and track slab warping [6]. Existing studies show that the effects of various temperature loads, such as annual temperature loads, sudden temperature drops, and daily temperature loads, cannot be ignored [7; 8; 9; 10]. The ballastless track system in the natural environment is affected by the overall rise and fall of the temperature and the temperature gradient along with the structure height [11; 12; 13]. The former will cause expansion deformation, and the latter will cause flexural deformation of the track system. Chinese and international scholars have achieved much research work on the time-dependent temperature field law, mechanical properties and failure mechanism of ballastless track systems under temperature loads [14; 15; 16]. [PERSON] [17; 18; 19] et al. embedded temperature sensors in ballastless tracks on curved bridges, performed long-term monitoring of the temperature field, and obtained the time-dependent temperature field law of longitudinally connected ballastless slab track-bridge systems under various natural conditions, such as extreme summer and cold winter temperatures, as well as the vertical and horizontal temperature gradient load modes. Based on meteorological principles, [PERSON] [20] established a three-dimensional finite element analysis model of the CRTS-II ballastless track temperature field and studied the time-dependent law and the influence of various factors on the temperature field of ballastless tracks. Based on the interface cohesive zone model, [PERSON] et al. [21] developed a three-dimensional finite element model of slab track and revealed the temperature characteristic of the slab track to studied the deformation and interface stress of the slab track under daily changing temperature, which provided information to the control of early interface damage. [PERSON] et al. [22] established a calculation model of the temperature warping stress according to the temperature measurement results of the track slab and analysed the warping stress and displacement of the track slab. [PERSON] et al. [23] established a stochastic model of the environmental seasonal temperature influence on the structure, which can accurately predict the frequency change of bridge structures under natural temperature changes and provides a basis for the reliable research of train-bridge interaction problems. [PERSON] et al. [24] studied the vertical and lateral temperature distribution of the track-bridge system under the shadow by establishing a finite element model and formula fitting, which provided a reference for the formulation and improvement of relevant specifications. In summary, most of the existing studies use finite element software for numerical simulation, and there are few related experimental studies; thus, it is difficult to fully consider the actual interlayer contact characteristics. In addition, the existing research has mainly focused on the temperature field of the track system and the mechanical properties of the track slab. The laws of the strain and relative displacement between the track layers under the action of natural temperatures are still not addressed. Most of the researches were carried out on a single structural layer to study its warping deformation law and damage mechanism, without fully considering the beam body, shear cogging and its own restraint effect. Furthermore, there is no systematic discussion on the mechanical properties and the cooperative performance of the CRTS-II ballastless track system on the bridge under natural conditions.