## 4 Discussion  
The measured temperature gradient load mode of this experiment is consistent with reference [18], when the thermal load is applied to the track structure, the load is gradually reduced from the top to the bottom of the track system with the highest value within the track slab. The load response is transferred layer by layer through the interlayer bond to achieve the harmonized deformation of the track structure. The interlayer gap between the track slab and the CA mortar weakens the bond between the layers, and the thermal load transferred from the track slab to the lower structure decreases  
Figure 23: Load–deflection curve of the concrete box beam.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Structural** & **Section** & **Before Test** & **After Test** & **Change Rate** \\ \hline \multirow{3}{*}{Track slab} & Fixed end & 31.11 & 27.01 & \(-\)13.18\% \\  & Mid-span & 23.81 & 18.45 & \(-\)22.52\% \\  & Sliding end & 37.17 & 32.83 & \(-\)13.21\% \\ \hline \multirow{3}{*}{CA mortar} & Fixed end & 19.55 & 17.29 & \(-\)11.58\% \\  & Mid-span & 35.91 & 33.11 & \(-\)7.78\% \\  & Sliding end & 38.81 & 37.03 & \(-\)4.57\% \\ \hline \multirow{3}{*}{Track bed} & Fixed end & 10.15 & 8.73 & \(-\)14.02\% \\  & Mid-span & 13.40 & 11.53 & \(-\)14.00\% \\ \cline{1-1}  & Sliding end & 12.30 & 12.53 & 1.89\% \\ \hline \hline \end{tabular}
\end{table}
Table 3: Overall stiffness degradation of the track structure after thermal loading.  
accordingly, subjecting the track slab to a larger thermal load magnitude. Therefore, under the same temperature load, with the increase of the number of thermal loading cycles, the gap initiates and gradually expands, resulting in the increase of the camber on the track structure, the decrease of the camber of the concrete box beam, the increase of the track slab strain, and the decrease of the CA mortar and the track bed strain. As the distance to the gap increases, the influence of the gap on the track structure strain is gradually weakened.  
Under the same thermal loading action, the displacement of the track structure is inversely proportional to its stiffness. The displacement gradually increases with the repeated action of the thermal loading, indicating that the stiffness of the track structure degrades gradually. The above analysis shows that during the test, the stiffness of the track structure is gradually degraded in the three stages of initiation-expansion-stabilization, and the rate of stiffness degradation can be calculated as follows:  
\[\mathrm{K_{1}=P/\delta_{1}\ ;\ K_{2}=P/\delta_{2}} \tag{1}\]  
\[\Delta\mathrm{K=}\frac{\mathrm{K_{1}-K_{2}}}{\mathrm{K_{1}}}=\frac{\delta_{2 }-\delta_{1}}{\delta_{2}} \tag{2}\]  
where, K\({}_{1}\) and K\({}_{2}\) are the structural stiffness values before and after the thermal loading test; P is the vertical static load; \(\delta_{1}\) and \(\delta_{2}\) are the structural displacements corresponding to the first and the 18 th thermal loading cycle, respectively; and \(\Delta\mathrm{K}\) is the stiffness degradation rate. When the track slab top surface temperature is increased to 30 \({}^{\circ}\)C, it can be seen from the above formula that after the 18 th thermal loading cycle, the stiffness of the track structure decreases by 14.96%.  
The stiffness of the track system is characterized by the slope of the relative displacement plot between the track system and the concrete box beam. The stiffness of the track-bridge structural system is characterized by the slope of the load-deflection plot of the track-bridge structure system, as shown in Table 4. After 18 cycles of temperature loading, the stiffness of the track structure was reduced by 14.96%, while the overall stiffness of the ballastless track-bridge structure system was reduced by 2.52%.