## 6 Discussion of The Bridge and Tunnel Characteristics  
The specific behaviours of the tunnel and bridge tracks from the measurements can be combined with the theoretical results in [1]. It has been shown that the bridge resonances would also be dominant in the ground vibration, see also [13]. Special bridge resonances were found in the time histories and narrow-band spectra in Section 4.2, which are strongly influenced by the speed-dependent axle sequence spectra of the train. The first torsional resonance at 7 Hz and the second bending resonance at 11 Hz have been amplified by the frequency range related to the axle distance in a bogie. The first bending mode of this 45 m long bridge is at 3.5 Hz whereas the standard viaduct bridges in China and Taiwan with a shorter span length of 32 m and 30 m, respectively, have their first bending eigenfrequency at about 5 Hz according to [14; 16]. In [16; 21], measured and calculated ground vibrations are shown for 100 and 200 m distance which have their dominant frequencies below 10 Hz and show some resonance effects for train speeds between 220 and 300 km/h. For lower train speeds of 120 to 200 km/h, a maximum at 16 Hz has been found in [19] which dominates the ground vibrations and is probably due to the second bending resonance of the standard Chinese viaduct. Bridge resonances and their effects on the ground vibrations have also been found for freight trains [20] and for local trains [21]. The bridge response has often been measured for long high-speed trains and the regular car-length frequencies dominate the Fourier spectra of the bridge and ground vibrations. Whereas in [21], the first to third car-length frequency is dominant in the far field velocities, the near field accelerations in [17] showed dominant peaks at the 7 th, 9 th-11 th and 13 th car-length frequency. These high-speed measurements confirm the strong effect of the axle sequence spectrum on the bridge response which has been stated for the present measurements of normal train speeds. The short continuous viaduct bridges in Japan have higher bridge resonance frequencies so that only the axle sequence peaks can be found in the low-frequency range [18]. For the tunnel line, the theoretical results in [1] show a reduction effect at high frequencies due to the excitation at depth and the wider load distribution across the tunnel invert. Therefore, the strong high-frequency amplitudes in the measurements can only be explained by a stronger excitation. The detailed analysis in Section 4.1 and in Appendix A show no special eigenmodes of the track slab in this frequency range, but the impact response shows a reduced radiation damping of the slab track with soft rail pads (Figure 15c) so that the combined wheelset-track resonance and the sleeper-passage component are stronger on the tunnel line. A reduction effect of the tunnel line at mid frequencies has been clearly measured by BAM and other institutes. This cannot be explained by the theoretical reduction in [1] which starts at higher frequencies. A smaller excitation due to smaller irregularities of the slab track would be a possible explanation, but this holds for the whole low-frequency range and not only for the specific mid-frequency range which has been observed in all measurements. The soil at the tunnel depth could be stiffer than at the surface and result in smaller amplitudes, but it is not probable that this holds for all measurements that have been reported here. Moreover, the strong mid-frequency ground vibration component on the surface line cannot be found in the axle-box accelerations in Figure 10, so there are no dynamic loads which could excite this component. It must be concluded that the mid-frequency ground vibration is excited by the static loads. The moving static loads yield axle impulses on the track which would superpose to the quasi-static response of the near-field soil with very low frequencies (Figure 7f).