## 1 Introduction  
With the rapid development of urbanization and the growing urban population, subways have become a critical component of urban planning [1, 2, 3, 4]. By the end of 2023, 55 cities in mainland China had opened a total of 10,879 km of operational subway routes [5]. As the railway network expands and train velocities increase, the associated environmental vibration issues become increasingly severe [6]. The vibrations stemming from the prolonged operation of the subway impact the structural integrity of neighboring buildings and directly influence the daily lives of residents [7, 8]. Nearly 700,000 people in Tehran, the capital of Iran, are exposed to noise pollution levels exceeding 70 dB, which could lead to significant dissatisfaction among residents in these areas and, in the long term, potentially result in hearing damage. The equivalent noise levels and maximum noise levels generated by rail transit within 20 m of residential areas are approximately 20 dB higher than the permissible noise levels [9, 10]. Onsite measurement is one of the most direct methods for detecting environmental vibrations induced by subway operations [11; 12]. Several researchers have studied the mechanism of structural vibration caused by rail transit vehicles and the propagation mode of vibration waves at the surface and underground levels [13; 14]. Similarly, [PERSON] et al. [15] and [PERSON] et al. [16] conducted extensive field observations on buildings near traffic arteries in some areas of Norway. Their findings underscored the substantial impact of rail transit-induced environmental vibrations on the structural integrity of adjacent structures. [PERSON] et al. [17] and [PERSON] et al. [18] conducted ground surface measurements induced by train operations in the elevated section of the transportation mainline. They derived conclusions regarding the acceleration vibration levels at observation points and their attenuation over distance through time history and spectral analysis. [PERSON] et al. [19], [PERSON] et al. [20], and [PERSON] et al. [21] also undertook extensive field measurements to investigate environmental vibrations near traffic arteries. [PERSON] [22] conducted a series of actual measurement experiments to investigate the ground vibration characteristics and vibration propagation attenuation rules caused by vehicles. Additionally, [PERSON] et al. [23] examined the characteristics and propagation rules of free surface vertical vibration around urban railways. This study aims to reveal the propagation mechanisms and attenuation laws of ground environmental vibrations induced by train operations in suburban railway underground sections, particularly in adjacent bidirectional tunnel structures. Existing research has primarily focused on the impact of single tunnel structures on vibrations, with limited studies on the vibration propagation characteristics in adjacent bidirectional tunnel structures (i.e., tunnels for the down-line and up-line tracks) [24; 25]. This gap restricts the optimization of vibration control measures in complex tunnel configurations, especially in suburban railway environments characterized by high-speed operations and high construction standards. The selection of the Taizhou S1 Line underground section as the study object is due to its typical bidirectional tunnel structure, high-speed operation, and stable geological conditions, making it a representative case for studying high vibration intensities. This study not only provides valuable insights for vibration control in suburban railways in China but also offers references for the design and optimization of similar rail transit systems worldwide. With the acceleration of global urbanization, understanding the impact of adjacent bidirectional tunnels on vibration propagation is crucial for engineers and planners to design more scientifically effective vibration reduction measures, ensuring the safety of urban infrastructure and the comfort of residents.