## 5 Conclusions  
In this study, the interlayer-debonding monitoring of a new-type track structure, CRTS-II-slab ballastless track, was investigated for supporting debonding identification and assessment. The monitoring framework targeting this issue was built in this study, which included three parts: (i) a monitoring scheme with instantaneous baseline and track-side sensor deployment near the debonding-prone area to facilitate interlayer debonding detection; (ii) a fiber optic system used for implementing the monitoring scheme and continuously capturing the low-frequency small-amplitude acceleration at the measurement positions; and (iii) multiple features characterized by easy implementation and fast calculation and a new indicator combining debonding-sensitive features for identifying and assessing interlayer debonding. The monitoring scheme with the design of mutual instantaneous baselines and trackside sensor deployment near the bonding-prone area was developed to facilitate its interlayer debonding detection and diminish the effects of varying environmental and operational conditions on feature and indicator comparison. With the application of the monitoring scheme to the CRTS-II-slab ballastless track on a viaduct in an in-service high-speed rail line, field monitoring data were continuously acquired by a high-sensitivity fiber optic monitoring system over the past two years. After validating the effectiveness of setting two identical segments of the track as mutual instantaneous baselines, multiple feature-extraction techniques characterized by easy implementation and rapid calculation were employed to differentiate the track segment with interlayer debonding from another track segment in a normal state. Six features in the time domain, two features in the frequency domain, and more than thirty features in the time-frequency domain were extracted. The reasons associated with specific vibration characteristics were explained. By performing seven feature fusion methods, it was found that Scheme 2 exhibited high sensitivity to debonding. Moreover, a new indicator, combining multiple debonding-sensitive features by similarity-based weights normalizing the initial difference between mutual instantaneous baselines, was developed to support \begin{table}
\begin{tabular}{c c} \hline \hline
**Scheme** & **Sensitivity** \\ \hline
1 & 2.87 \\
2 & 3.58 \\
3 & 2.23 \\
4 & 2.67 \\
5 & 2.01 \\
6 & 1.99 \\
7 & 1.32 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Sensitivity of seven features fusion methods. Figure 23: Variation of interlayer debonding and Euclidean distance with time. rational and comprehensive assessment of interlayer debonding quantitatively. Euclidean distance between the indicator curves of the two track segments of the seven features fusion methods both had a continuous increase, which was coincident with the occurrence and deterioration of interlayer debonding. This study is conducive to the establishment of an effective feature pool and a new indicator for rapid debonding identification and assessment of track system and thus contributes to the safety of train operation and the comfort of passengers. Extensive investigations into the SHM of the ballastless track exist, but the study on this type of interlayer-debonding monitoring scheme and its application to the debonding identification of an in-service HSR track with the assistance of the new indicator is rarely reported. The difficulties for generalizing this work to other applications are the establishment and validation of mutual instantaneous baselines and the development of a damage-sensitive indicator with the mechanism able to normalize initial error between the mutual instantaneous baselines. Conceptualization, G.G. and J.W., Methodology, J.W., Formal analysis, G.G., Investigation, G.G., Resources, G.G., Writing-Original draft, G.G., Writing--Review & Editing, G.G. and J.W., Supervision, B.D. and Y.D. All authors have read and agreed to the published version of the manuscript.