## 5 Results and Discussion
### Comparison of t2 and T2  
#### 5.2.1 Sleeper  
Figure 14 compares the displacement and velocity of sleepers between t2 and T2. Monitoring points S2 and S4 are the points of trains load in T2. Displacement and velocity of the two monitoring points (S2 and S4) of the t2 sleeper basically coincide, and the displacement and velocity of the T2 sleeper also coincide. Therefore, four monitoring points are compared in this figure, while only two curves can be seen intuitively in each sub-figure.  
\(D_{\rm H}\) of t2 and T2 oscillates in the left and right directions during train vibration (Figure 14a). The amplitude of \(D_{\rm H}\) of t2 and T2 gradually increases between 0 s and 1.3 s, and then the amplitude of \(D_{\rm H}\) basically remains unchanged. After that, the peak-peak values of the \(D_{\rm H}\) of t2 and T2 are very similar. Therefore, the \(D_{\rm H}\) of t2 and T2 are basically not affected by the size of diameter, and the peak-peak value of t2 and T2 is less than 0.35 mm. The \(D_{\rm H}\) of t2 and T2 has a small response in the train vibration process.  
Compared to Figure 14a,c, \(V_{\rm H}\) of T2 is smaller than that of t2. The dynamic response of T2 in the horizontal direction is smaller than that of t2, and T2 is more stable than t2 in the horizontal direction.  
\(D_{\rm V}\) of t2 and T2 oscillates in the upward and downward directions during train vibration (Figure 14b). \(D_{\rm V}\) of t2 is much larger than that of T2. The peak-peak value of the \(D_{\rm V}\) of t2 is about 1.9 mm, and the peak-peak value of the \(D_{\rm V}\) of T2 is about 1.1 mm. Therefore, the vertical displacement of t2 is about two times that of T2.  
The amplitude of the \(D_{\rm V}\) of the t2 gradually increases between 0 s and 2.7 s, and then the amplitude of the \(D_{\rm V}\) basically remains unchanged. Therefore, the t2 steeper reaches a new equilibrium state at about 2.7 s, and the t2 tends to be in dynamic equilibrium. The vibration amplitude of the \(D_{\rm V}\) of the T2 increases gradually from 0 s to 0.8 s and decreases gradually from 0.8 to 1.7 s. Subsequently, the vibration amplitude of the \(D_{\rm V}\) of the T2 is stable in the region. It can be seen that the T2 gradually reaches the equilibrium state between 0.8 and 1.7 s. Compared with the small-diameter tunnel, the time needed to reach the new equilibrium state in the train vibration process in the super-large diameter tunnel is shorter (2.7 s) than that in the small-diameter tunnel. This shows that the super-large diameter tunnel is more stable than the small-diameter tunnel in the train vibration process. Therefore, it can be seen that T2 is more stable than t2 from two aspects: the time of the tunnel reaches the equilibrium state and the amplitude of \(D_{\rm V}\).  
Compared to Figure 14b,d, the laws of \(D_{\rm V}\) and \(V_{\rm V}\) of t2 and T2 are basically similar. The peak-peak of the \(V_{\rm V}\) of the t2 is about 37 mm/s, and the peak-peak of the \(V_{\rm V}\) of the T2 is about 17 mm/s, and the \(V_{\rm V}\) of the t2 is about two times that of the T2.  
According to Figure 14a,d, the sleeper in t2 and T2 will occur the horizontal and vertical vibration displacement and velocity during the train running in T1. In general, the displacement and velocity of t2 are more significant than those of T2.  
The responses of \(D_{\rm V}\) and \(V_{\rm V}\) in t2 and T2 are greater than those of horizontal directions. The vertical response of the structure should be paid more attention to the process of train vibration. T2 is more stable than t2 from two aspects: the time of the tunnel reaches the equilibrium state and the vibration amplitude of the structure's dynamic and static response. The dynamic characteristics of the \(D\) twin tunnels are very different from those of the \(d\) twin tunnels, so the dynamic characteristics of the train vibration in the \(d\) twin tunnels cannot be directly applied to the \(D\) twin tunnels.