## 1 Introduction
Since track stiffness is an important parameter that affects the dynamic characteristics of the wheel-rail system; the purpose of studying LVT stiffness is to optimize the dynamic characteristics of the track under the premise of ensuring the safety and stability of heavy-haul trains [17]. In the design, starting from the track stiffness, the concepts and methods of track structure dynamic design are introduced to reduce or optimize the construction investment; in the maintenance, the existing maintenance procedures are appropriately adjusted around the detection and control of the track stiffness to reduce maintenance investment. In the research of track stiffness, it is necessary to reasonably determine the total track stiffness, the stiffness combination of the various components of the track and find the limit of the rate of change of the track stiffness according to the operating conditions [18; 19]. The reasonable value of track stiffness should firstly optimize the dynamic characteristics of the track, and secondly, it should be considered that less investment in construction and maintenance is a complex problem involving a wide range of aspects [20; 21]. At present, there are many studies about LVT on high-speed railways and subways, but there are few studies on the heavy-haul railway, and there are fewer references on the dynamic response of track structure parameters to heavy-haul vehicle-track systems [22]. Under such operating conditions, whether LVT can replace ballasted tracks or other types of ballastless tracks, the reasonable stiffness range of the LVT track structure, whether the geometry of the track structure can be maintained under the effect of train load, and whether the safety and comfort of driving can be guaranteed are problems that still need to be researched. Furthermore, the object of the present study was to investigate traditional speed, acceleration or displacement responses, but system dynamics response sensitivity features are rarely discussed [21]. In light of this, we calculated and compared the system dynamics of the upper heavy-haul vehicle/track system based on system dynamics response sensitivity and vehicle-track coupled dynamics under the influence of different track structure parameters, using LVT on the heavy-haul railway as an example. The appropriate under-rail stiffness and under-block stiffness are suggested from the perspective of system dynamics response sensitivity, in order to provide scientific guidance for the parameter design of LVT on heavy-haul railways in the future. Based on the existing research on LVT vibration damping mechanism, indoor tests, construction technology, etc. [11; 12; 13; 14], we incorporated LVT in a tunnel to focus on the suitable stiffness of this track structure under the operating conditions of a 30 t heavy-haul train. The time domain and frequency domain analysis methods of the wheel-rail system were used in a dynamic analysis to analyze the dynamics effect of the track stiffness based on the analysis of the rule of the track's dynamic parameters with the goal of optimizing LVT dynamic characteristics. Sensitivity to the stiffness of the optimization analysis method was also analyzed to study the value of the total stiffness and the reasonable combination of the stiffness of the heavy-haul railway LVT.