## 1 Introduction  
It has been almost 60 years since the occurrence of the first high-speed railway (HSR) line. Countries around the world are developing and constructing HSR on a large scale because of its smoothness and comfort [1]. In the process of long-term service, the track structure of HSR deteriorates inevitably due to external influence, which directly affects the track quality. Therefore, it is essential to realize the scientific and efficient maintenance of the track structure to ensure the reliability of HSR. In all kinds of HSR track structures, slab track, with advantages of high regularity, strong durability, and low maintenance costs, has been widely utilized in the world [2]. Especially in China, slab tracks are adopted in most of the HSR lines [3]. During the long-term operation of the track system, complicated temperature conditions are the main factors for the service state of the slab track [4, 5]. Continuous high temperature and large temperature gradient can cause structural deterioration, weaken the interface constraints, and even induce a series of damages [6, 7]. Moreover, repeated train impact can also accelerate the development of the damages [8, 9]. Due to the complexity of external factors,the performance of the slab track is temporally and spatially variant, which makes the occurrence of structural damages unexpected and brings challenges to damage detection. The common temperature-induced damages of slab track include interface debonding of track slab and cement asphalt mortar (CAM) layer [10; 11], property deterioration of CA mortar [12; 13], defects of wide and narrow joints [14; 15], and arching deformation of track slab [16; 17]. Among them, slab arching deformation has attracted much attention as the most prominent damage to the slab track [18]. The interlayer damages and joint defects can change the vertical and longitudinal structural connection states, which reduce the stability of the slab track. In the high-temperature environment in summer, the track slabs with the above defects are prone to eccentric compression and occur instability, thus causing slab arching [19]. Serious arching deformation results in the mechanical property degradation of the track structure, the generation of addition track irregularities, and the increase in vehicle vibration [18; 20]. Therefore, as the first and the most critical step in the treatment of slab arching damages, the accurate and rapid detection of the damages is of vital significance for the state maintenance of track structures and the long-term service of HSR. At present, the state information of the slab track is lacking, and manual inspection [21] is mainly used to detect slab damages. The inspectors measure the sizes of on-site interlayer gaps by a variety of measuring tools, including feeler gauges, dial indicators, and levels [22], and then make maintenance plans by comparing the measurements with standards. However, this kind of detection method is inefficient and inadequate. Due to the high operating density of HSR, the skylight time for inspection is short. This method needs to be carried out frequently to maintain the health state of track structure, which costs a great deal of workforce and resources. Accordingly, there is an urgent need to establish an economical and efficient arching detection method. Currently, researchers have made some explorations in the damage detection of slab tracks. Most of them inspected the internal defects (e.g., cracks and voids) and interlayer gaps of track structures by non-destructive detection techniques. The detection methods utilized include impact-echo method [23; 24], ground-penetrating radar method [25; 26], and modal testing method [27]. Some researchers also utilized distributed optical fiber sensors to monitor the deformation of track structures [28; 29]. Nevertheless, the methods mentioned above still have some limitations: (1) the non-destructive detection methods are mostly employed in fixed-point testing and are time-consuming and laborious.