## 2 Monitoring Scheme  
### Instantaneous Baseline Establishment for Comparative Monitoring  
Railway monitoring in the field is closely related to the form of track structure and foundation [48], temperature loads [49], train loads [50], and other factors. After communicating with the railway management and conducting on-site research and surveys for more than half a year, the authors found that two different segments in the same line at a distance of approximately 3 km had different chances to debond. Segment 1 was in a good state without debonding, while Segment 2 was in a poor state with slight debonding. The form of track structure and foundation are identical in two segments eliminating monitoring errors caused by differences in the form of structure. The track structure, which is composed of concrete, could lead to different temperature stresses when it is subjected to different temperature loads because of the thermal expansion and contraction effect, and therefore temperature loads have a significant impact on the monitoring. The monitoring errors caused by temperature loads may be introduced if the same zone is monitored at different times. In order to eliminate the influence of temperature loads on the monitoring results, comparative monitoring is carried out in two segments simultaneously. Due to the large variation in loads from one train to another, the use of comparative monitoring of two segments ensures consistency of train loads. In summary, the two segments are monitored simultaneously can eliminate errors introduced by structural differences, temperature loads, and train loads. At the beginning of the monitoring period, the differences between the parameters of the two segments were not significant, and as the monitoring time increased, the differences between the parameters of the two segments were large. This phenomenon indicates that Segment 1 is still in a good state, while Segment 2 has suffered from an interlayer debonding defect. As of May 2021, the track structure in Segment 1 was found to be still intact after a site survey, indicating that the monitoring system proposed in this paper was effective within a limited period of time. Instead of using the conventional feature-comparison concept, which contrasts the current value of a feature with the historically pre-stored value of the same feature, this study builds an instantaneous baseline for diminishing the effects of varying environmental and operational conditions during the comparative analyses of debonding-associated features. Two CRTS-II-slab ballastless track segments of a standard simply-supported box-girder concrete viaduct in an in-service HSR were selected. Their cross-section sketch and longitudinal sketch are shown in Figures 1 and 2. Figure 1 displays the cross-section of CRTS-II-slab ballastless track on a viaduct which is composed of rail, track slab, CA mortar layer, supporting concrete layer, and box-girder. Figure 2 shows three measurement positions of each track segment. Figure 1: Cross-section sketch of CRTS-II-slab ballastless track structure on the viaduct. The two segments were intended to serve as a mutual instantaneous baseline for each other in feature comparison because they have the same structure and boundary conditions. In the early stage of monitoring, both Segment 1 and Segment 2 were in normal state, while Segment 2 seemed to have minor and negligible marginal debonding between track slab and CA mortar layer. The on-site monitoring system was implemented in January 2019. The early stage of monitoring included the first half-year in 2019, and the current stage of monitoring mentioned in this paper was from early October 2020. The vibration characteristics of Segment 1 and Segment 2 were analyzed to examine the effectiveness of instantaneous baseline. The high similarity between their vibration characteristics demonstrated that the two selected segments can be an instantaneous baseline for each other.