## 1 Introduction  
In recent years, with the rapid development of tourism and the improvement in the awareness of natural landscape protection, more and more people have begun to think about how to develop the tourism industry while protecting the original style of the scenic spot and adapting to the complex and changeable terrain conditions. Therefore, the straddle-type monorail tour transit system (MTTS) has gradually been popularized. As a new type of rail transit, it has the advantages of a small land area, short construction period, strong climbing ability, and strong terrain adaptability [1, 2, 3, 4]. More than 30 projects have been completed or are under construction in China, and typical projects are shown in Figure 1.  
Unlike the traditional monorail systems and railways, most MTTS have adapted elevated steel structures with lightweight beams and heavy-weight vehicles, showing the characteristics of small dead loads and large live loads. Therefore, the dynamic response of the track structure during vehicle operation may be notable [5, 6, 7, 8]. The tracking subsystem of MTTS is composed of a track structure and a maintenance and evacuation platform for relevant personnel. The track structure bears the vehicle load and acts as a rail. Different from the traditional wheel-rail contact, the wheel-rail contact of the MTTS is a rubber wheel contact, which also makes the research results of the traditional railway unable to be applied to the research of MTTS [9; 10; 11]. With the development of MTTS towards long-span use, the dynamic interaction problem of track structure is becoming highly prominent. While taking the vehicle or walking on the evacuation platform, a notable vibration response may be present, which may cause human discomfort or even reduce the safety and service life of the structure [12; 13; 14]. Thus, reducing the system's dynamic response [15; 16; 17; 18; 19; 20; 21; 22; 23] and improving the comfort of humans [24; 25; 26; 27] have been researched topics of great concern. For the MTTS, the crowd belongs to an eccentric excitation force on the structure, and the periodic lateral load will generate with walking, resulting in the lateral vibration of the system. This problem has not received enough attention from engineers. In recent years, some large-span structures have also experienced evident lateral vibrations caused by pedestrians. The Millennium Bridge in London, UK, has experienced a severe lateral vibration due to pedestrians, which has seriously affected pedestrian comfort and safety, resulting in a significant phenomenon of \"collective synchronization\" of walking [28]. [PERSON] [29] et al. considered people as a time-varying dynamic model. They established the coupling dynamic equation between the beam and the human to study the dynamic response of the people and the structure. The results showed that a person's natural frequency, walking frequency, and speed were important factors affecting the structure's dynamic response and the person's comfort. Many studies have also analyzed structural vibration and its impact on comfort and studied methods to reduce vibration and improve comfort [30; 31; 32]. The technical code for urban pedestrian overpasses and pedestrian underpasses [33] is the main code for designing pedestrian bridges in China. The frequency-adjustment method is adopted to consider the vertical vibration of the structure, and the vertical frequency is required to be no less than 3 Hz. However, the code does not provide further suggestions or methods for evaluating vibration serviceability that cannot meet the code's requirements, and the problem of lateral vibration is still unresolved. Although there are differences between MTTS and pedestrian bridges, the research results of pedestrian bridge vibration have significance for the research on pedestrian structure vibration of the evacuation platform of the MTTS.