## 1 Introduction  
High-speed railways (HSRs) are flourishing in various countries owing to their efficiency, convenience, comfort, and low maintenance and repair workload [1]. By the end of 2020, China's HSR mileage reached 38,000 km [2], ranking first in the world. A ballastless track slab is an important part of various types of track structures, as it plays a \"top-down\" role (i.e., bearing the train load and transferring it down to the foundation) [3]. However, prolonged operation could cause various defects in track slabs, such as warping deformation of track slab [4, 5], wide and narrow joint damages [6, 7], interlayer debonding [3, 8, 9], and arching deformation of track slab [10, 11, 12]. Under temperature load [13, 14] and high-frequency train load [14], these damages may cause track slab deformation and abnormal vibrations in wheel-rail system. Considering the complex external factors, the performance of track slab is spatially and temporally variable, so the occurrence of track slab damage is sudden and unpredictable. Timely detection of track slab defects prevents track slab failure and ensures safe train operation. Therefore, the real-time monitoring of track slab performance and the identification of the damage condition of track slabs are essential for long-term safe operation of the HSR. Existing track slab defect detection methods can be divided into three categories: visual inspection [15; 16], nondestructive testing (NDT) [17; 18; 19; 20; 21; 22; 17], and structural health monitoring (SHM) [23; 24; 25; 26; 27]. Most of the visual inspection methods rely on a variety of track inspection equipment, such as road rulers, gauges, and various customized equipment. Although visual inspection methods are flexible and simple, they have limitations that most of them require working on the track structure and must be conducted in skylight. HSR skylight time is short and it is difficult to detect the track structure damage in a short time by using this method. By contrast, NDT methods mainly use noncontact measurement to detect track defects. Some scholars use the differences between rail inspection vehicles' static and dynamic inspection data [17] and the geometric data [4; 11] to detect track slab deformation. The track inspection data are derived from various sensing devices installed on the track inspection car. However, owing to the high cost and limited number of track inspection vehicles, track inspection vehicle measurement cycle of approximately half a month can be conducted, resulting in a long detection cycle and a large number of missing effective data. In addition, according to the field survey results of railway management department, track slab deformation mostly occurs during daytime and tends to be stable at night. However, most of the track inspection data in track slab deformation detection methods are collected at night [28]; thus, the information during the day, which is significant for the safe operation of the track, is omitted. Therefore, such track slab deformation detection methods have limitations. In addition, some scholars used the impact-echo waveform [18; 19; 20] and ground penetrating radar [21; 22] methods to detect track slab defects. However, these methods require on-site inspection by technicians for a specific section of track, resulting in delayed detection of damage, which not only takes up skylight time but also affects the normal operation of high-speed trains. Therefore, there is an urgent need for a real-time monitoring method for track slab defects that can monitor without affecting the normal operation of trains. Therefore, with the aid of advanced sensing technology, real-time monitoring methods for track structure defects have been developed. Some approaches used fiber optic sensors for the real-time monitoring of track structure deformation caused by temperature [23], and for measuring the lateral displacement of the track structure [24].