## 1 Introduction
Some of the investigations, including the NDT using IE [18] and GPR [14] methods, belong to the partners/colleagues of the authors in this paper, and their experiments were conducted on a CRTS-II-slab track model with mortar defects. The NDT techniques are generally performed intermittently by technical personnel or inspection vehicle going to the field for examining specific sections of track, leading to the late awareness of damage and the occupation of skylight time or the interruption of the normal operation of the high-speed train. Structural health monitoring, supporting timely detection of damage, and exempting the interruption of the normal operation of a monitored system (e.g., track-vehicle system), has been widely applied to different civil structures [19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37]. Examples in railway engineering without the interruption of the normal operation of the train include the interval dynamic analysis of the train-bridge system [23], rail crack identification [24], wheel damage detection [21; 25], and track condition monitoring [26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37]. The studies on track condition monitoring involve track slab deformation measurement by using computer vision [33; 34], dynamic strain and acceleration [27], and displacement measurement [28] by remote sensing, measurement of different parameters (such as stress and sound wave) by using wireless sensor networks [29], deformation measurement by using fiber Bragg grating (FBG) sensors [30; 31; 32; 33], or distributed fiber optic sensors [34; 35], track axial force measurement by using FBG sensors and electronic sensors [36], and geometrical rail track condition monitoring by using FBG and Raman distributed temperature sensors [37]. Both fiber optic sensors and piezoelectric sensors are commonly used in SHM. As the voltage of HSR can reach thousands or even tens of thousands of volts, traditional piezoelectric vibration sensors are potentially subjected to great electromagnetic interference. Fiber optic sensing technology [38] takes light waves as a carrier and fiber as a medium, which hasthe advantages of anti-electromagnetic interference, corrosion resistance, long-distance transmissibility, strong multiplexing, and networking ability, and thus meeting the needs of on-site monitoring of rail track. The partners of the authors in this paper have developed interferometric fiber optic accelerometers for vibration measurement [39]. These sensors were used for measuring the vibration of a CRTS-II-slab track and investigating the relationship between temperature and track vibration [40]. Different from the previous investigations, a monitoring scheme that facilitates interlayer debonding detection of in-service CRTS II slab track and diminishes the influence of environmental and operational conditions on feature and indicator comparison is developed in this study. Fiber optic vibration sensors [39] are deployed in a cable trough because in the region allowed for sensor placement, the cable trough is the transversely nearest place to the debonding-prone area. Conventionally, feature comparison between the situations with and without damage is based on the comparison of the current feature to a pre-stored baseline built when the structure was in its pristine state. This comparison between current data and past pristine baseline data (i.e., the subtraction between them) can bring complications in data management and the inability to accommodate the effects of varying environmental and operational conditions, followed by false alarm or undetected damage. Therefore, an instantaneous baseline, which can be a series of geometrically identical wave-transmission paths designed for crosschecking the path health conditions by using current data, was developed [41; 42]. Similarly, the instantaneous baseline built in this study is designed to diminish the effects of varying environmental and operational conditions on feature comparison and indicator comparison.