# Influence of Aging Conditions on the Dynamic Stiffness of EPDM and EVA Rail Pads
## 1 Introduction  
Railway has been, since its inception, one of the main modes of transportation, thanks to its ability to move large volumes of both passengers and goods efficiently and sustainably. Specifically, within the European Union, the railway sector stands out as a benchmark for environmental sustainability. It is the only mode of transport that has consistently reduced greenhouse gas emissions since 1990. In this context, rail is responsible for transporting 13% of freight and 7% of passengers, while its emissions account for only 0.4% of the total emissions from the transport sector [1]. Additionally, the sector has demonstrated steady growth, with annual increases of 2.5% in passenger traffic and 4.1% in freight transport [1]. According to the Rail Market Monitoring report by the European Commission [1],38.7 billion euros were invested in rail infrastructure in 2018, with 53% of that investment dedicated to maintenance and renewal, particularly of the track superstructure. The railway superstructure is a composite system of various elements responsible for transmitting the dynamic load of vehicles to the ground, ensuring passenger comfort, and reducing vibrations and noise emissions. The components that comprise the railway superstructure include rails, clips, bolts, dowels, ribbed plates, and rail pads, as illustrated in Figure 1. In the ballasted track typology, the system is fixed to sleepers resting on the mass of the ballast, while in the slab track typology, the system is attached to the concrete slab. The vertical stiffness of the track is a crucial variable that significantly impacts the quality of transportation, as well as its maintenance and durability. In ballasted tracks, the vertical stiffness of the assembly is jointly determined by the stiffness of the ballast and the rail pads [2], while in slab track systems, the vertical stiffness depends almost entirely on the rail pads [3]. This finding is supported by the study by [PERSON] et al. [3] in which the authors employed Finite Element modeling to investigate the dynamic behavior of high-speed tracks. The pioneering study by [PERSON] and [PERSON] in 1982 [4] aimed to identify the cause and recommend a solution for the development of rail seat bending cracks observed in concrete sleepers on several tracks with service histories ranging from a few months to about two years. The major objective of this experimental study was to identify a value of the pad stiffness to prevent the initiation of cracks in sleepers subjected to the worst in-service impact conditions [5]. Subsequent studies were conducted to further understand the influence of the stiffness of the system. Specifically, previous research [5; 6] has noted that an increase in pad stiffness leads to a reduction in impact attenuation capability, thereby causing the deterioration of the sleepers and ballast. Insufficient stiffness, as documented in the literature [7; 8], has been reported to result in excessive deflection of the rail, higher energy consumption, and increased accelerations experienced by the vehicles. These findings provide valuable insights into the relationship between stiffness and the performance of high-speed tracks, which can inform the design and maintenance of these systems. Rail pads are essential components of railway track systems because they play a crucial role in improving load distribution, providing flexibility, damping the impact of loads, reducing noise and vibrations, and providing electrical insulation between rails [9]. The  
Figure 1: Example representation of a track on ballast. market offers a wide range of rail pad solutions, differing in size, constituent material (natural rubber, synthetic rubber, plastics, composites, etc. ), and surface geometry (plain, grooved, prismatic, and studded patterns). Typically, rail pads are rectangular in shape, measuring 180 mm in length, 140-150 mm in width, and ranging in thickness from 4.5 to 15.0 mm.