## 6 Conclusions  
This study further processed the ground vibration acceleration data affected by the bidirectional tunnel configuration in a suburban railway underground section, providing an in-depth analysis of the impact of adjacent bidirectional tunnel configurations on environmental vibration. The specific methods included extracting the peak acceleration at each measurement point, calculating the vibration acceleration levels and maximum displacement, and comparing and analyzing the peak acceleration, maximum vibration acceleration levels, and maximum displacement at each measurement point. Based on the analysis results, the following conclusions are drawn:1. The vibration frequencies at each measurement point are primarily concentrated in the 20-50 Hz range, with dominant frequencies around 25 Hz and 35 Hz. In the 6.3-25 Hz frequency range, the vibration acceleration level in the X-direction at the measurement point farthest from the tunnel consistently remains the smallest among all measurement points, corresponding to the position with the lowest peak acceleration in the X-direction. This indicates that the tunnel structure has an amplification effect on mid-to-high frequency vibrations, aligning with the research objective of exploring the impact of adjacent bidirectional tunnel configurations on train-induced environmental vibrations. 2. The high-frequency components of vibration in the Z-direction (vertical) disappear at P5 and reappear at P6, confirming the tunnel structure's role in reflecting and attenuating vertical vibration waves. In the X-direction (horizontal, perpendicular to the train direction), a significant increase in high-frequency components is observed at P3, verifying the amplification effect of the tunnel lining on vibration waves in the horizontal direction. As the distance from the tunnel increases, the expected attenuation effect of the soil on the power spectral density in the vertical 40 Hz-100 Hz frequency band is not evident. The tunnel structure significantly affects the propagation path and attenuation pattern of vibration waves through reflection and refraction. In the Z-direction, high-frequency vibrations disappear at specific measurement points, indicating that the tunnel lining material absorbs and reflects vibration energy. In contrast, in the X-direction, high-frequency vibrations are amplified, suggesting that the tunnel structure has an amplification effect on vibration waves in the horizontal direction. 3. It is recommended to avoid placing vibration-sensitive equipment in the area between the two tunnels, especially in regions where the peak acceleration in the X-direction is higher, to ensure the stability and accuracy of the equipment. When planning buildings above the tunnel, residential areas with strict vibration acceleration requirements should be avoided, or vibration isolation designs should be implemented where necessary to enhance the building's vibration resistance. In high-vibration zones, the use of elastic materials or vibration isolation pads for designing vibration isolation foundations is recommended to reduce the impact of high-frequency vibrations caused by the tunnel on the buildings. Despite the valuable findings of this study, there are several limitations. The vibration data were only collected from a representative section of the Taizhou S1 Line underground segment, without covering the entire railway line, which may affect the generalizability of the results. Environmental vibrations are influenced by various factors, such as geological conditions, climate changes, train speed, and operating frequency. Although some of these factors were controlled for, there may still be unaccounted-for interference factors that could affect the accuracy of the data and the reliability of the results. Although high-precision accelerometers and dynamic signal testing and analysis systems were used, along with strict calibration procedures, limitations of the equipment and potential calibration errors may still impact the measurement results.