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Published: 16 April 2025
Citation: Rivas, I.; Sainz-Aja, J.A.;
Ferreño, D.; Carrascal, I.; Casado, J.;
Diego, S. Influence of Aging
Conditions on the Dynamic Stiffness of
EPDM and EVA Rail Pads. Appl. Sci.
2025,15, 4394. https://doi.org/
10.3390/app15084394
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Article
Influence of Aging Conditions on the Dynamic Stiffness of
EPDM and EVA Rail Pads
Isaac Rivas *, Jose A. Sainz-Aja , Diego Ferreño , Isidro Carrascal, Jose Casado and Soraya Diego
LADICIM (Laboratory of Materials Science and Engineering), University of Cantabria, E.T.S. de Ingenieros de
Caminos, Canales y Puertos, Av./Los Castros 44, 39005 Santander, Spain; jose.sainz-aja@unican.es (J.A.S.-A.);
diego.ferreno@unican.es (D.F.); isidro.carrascal@unican.es (I.C.); jose.casado@unican.es (J.C.);
soraya.diego@unican.es (S.D.)
*Correspondence: rivasi@unican.es
Abstract: The railway sector plays a crucial role in sustainable transportation by reducing
greenhouse gas emissions while supporting an increasing volume of freight and passenger
transport. Rail pads, essential components in railway infrastructure, mitigate vibrations
and distribute loads; however, their long-term performance is influenced by environmental
and mechanical degradation, affecting track durability and maintenance costs. Despite
their significance, the degradation mechanisms impacting the dynamic stiffness of EPDM
(Ethylene Propylene Diene Monomer) and EVA (Ethylene Vinyl Acetate) rail pads remain
insufficiently characterized. This study examines the effects of mechanical and chemical
aging on the stiffness of these materials through 864 dynamic stiffness tests, analyzing
three types of rail pads under mechanical cycling (up to 2,000,000 cycles), UV (ultraviolet
light) exposure (100–500 h), and hydrocarbon exposure (100–500 h). Mechanical aging
increases stiffness across all pads, with Pad C (EVA) exhibiting the most pronounced
increase (27%). The effects of UV exposure vary by material, leading to a stiffness reduction
of up to 11.5% in Pad B (EPDM), whereas Pad C (EVA) experiences a 9.5% increase under
prolonged exposure. Hydrocarbon exposure also presents material-dependent behavior,
with Pad A (EPDM) experiencing an 11.5% stiffness reduction at low exposure but partial
recovery at higher exposure, while Pad C (EVA) shows a 5% increase in stiffness under
prolonged exposure. These findings offer valuable insights into the aging mechanisms
of rail pads and underscore the importance of considering degradation effects in track
maintenance strategies.
Keywords: dynamic stiffness; rail pads; polymer degradation; railway maintenance;
mechanical properties
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 trans-
port [
1
]. According to the Rail Market Monitoring report by the European Commission [
1
],
Appl. Sci. 2025,15, 4394 https://doi.org/10.3390/app15084394
Appl. Sci. 2025,15, 4394 2 of 16
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.
Appl. Sci. 2025, 15, x FOR PEER REVIEW 2 of 17
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 super-
structure.
The railway superstructure is a composite system of various elements responsible for
transmiing the dynamic load of vehicles to the ground, ensuring passenger comfort, and
reducing vibrations and noise emissions. The components that comprise the railway su-
perstructure 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 aached 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.
Figure 1. Example representation of a track on ballast.
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
Chen et al. [3] in which the authors employed Finite Element modeling to investigate the
dynamic behavior of high-speed tracks. The pioneering study by Dean and Harrison 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 aenuation capability, thereby causing the deterioration of the sleepers and bal-
last. 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 cru-
cial role in improving load distribution, providing flexibility, damping the impact of
loads, reducing noise and vibrations, and providing electrical insulation between rails [9].
The market offers a wide range of rail pad solutions, differing in size, constituent material
Figure 1. Example representation of a track on ballast.
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
Chen et al. [
3
] in which the authors employed Finite Element modeling to investigate the
dynamic behavior of high-speed tracks. The pioneering study by Dean and Harrison 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
Appl. Sci. 2025,15, 4394 3 of 16
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. The mechanical behavior of polymeric materials used to fabricate rail pads is
nonlinear and dissipative and is affected by several in-service conditions such as tempera-
ture, frequency, tow load, and axle load. In a previous study by Sainz-Aja et al. [
10
], three
types of rail pads fabricated using EPDM, TPE, and EVA were exhaustively characterized
(720 tests) under different combinations of the aforementioned factors to determine their
influence on the dynamic stiffness of the component. This experimental information was
subsequently modeled using Machine Learning algorithms. The analysis of feature impor-
tance revealed that temperature and tow load are the most influential variables affecting the
dynamic stiffness of EPDM and TPE rail pads, while tow load is the most significant feature
for EVA rail pads. Finally, in Sainz-Aja et al. [
11
], a Finite Element model of the slab track
was developed to generate synthetic samples representative of actual in-service conditions
where relevant track parameters were varied within their usual ranges using a Monte Carlo
procedure. The results were then used to train and validate a series of regression Machine
Learning models, which showed that the properties of the soil, characteristics of the rail
pads, and axle loads have the greatest influence on the railway infrastructure.
Polymer materials utilized in the railway superstructure are susceptible to degradation
under various environmental conditions [
12
]. Such conditions encompass low and/or high
temperature, exposure to rain and/or sunlight, water and/or soil properties, inducing
different kinds of degradation mechanisms such as oxidation, hydrolysis, ultra-violet (UV)
irradiation, chemical influence, aging or fatigue, among others. A paper authored by Sol-
Sánchez et al. [
7
] outlines the degradation mechanisms (physical–chemical and mechanical)
experienced by rail pads exposed to the railway environment. The authors reported that
environmental factors can cause the pads to progressively increase in stiffness, with values
reaching up to 33–41% for a service life of 1–3 years, respectively.
There is a recent bibliography on the aging and changes in the mechanical behavior
of the materials used in this study. In this sense, Tayefi et al. [
13
] show that under accel-
erated thermal aging, EPDM undergoes oxidation at high temperatures (120–140
◦
C) that
generates hydroxyl and carbonyl groups, reducing its crystallinity and deteriorating its
mechanical properties. Likewise, Xin-Yi et al. [
14
] investigate
γ
-ray irradiation, finding that
it reduces the Mooney viscosity and molecular weight of EPDM, increases the gel content,
and raises the glass transition temperature, which modifies its internal structure and pro-
cessing performance; finally, Shuang-Hong et al. [
15
] examine the thermo-oxidative aging
of EPDM, noting that crosslinking predominates during degradation, which translates into
an increase in hardness and tensile strength along with a decrease in permanent defor-
mation. In the case of EVA, Oliveira et al. [
16
] present a review in which the degradation
of the EVA encapsulant used in photovoltaic modules is described under the influence
of environmental factors (temperature, humidity, and UV radiation), affecting its chem-
ical, mechanical, optical, and electrical properties, which in turn reduces the system’s
efficiency and durability. Other authors report similar results; for example, Ji et al. [
17
]
show that during thermo-oxidative aging at 165
◦
C in air, EVA degrades mainly due to
the decomposition of its acetate groups, leading to significant changes in its electrical and
mechanical properties.
This research aims to experimentally evaluate how mechanical and environmental
deterioration conditions affect the dynamic stiffness of rail pads manufactured from EPDM
and EVA materials. Based on the literature review, it has been established that there
are existing studies addressing the aging and changes in the mechanical behavior of
Appl. Sci. 2025,15, 4394 4 of 16
EPDM and EVA materials under various environmental conditions [
13
–
17
], as well as
research identifying specific degradation mechanisms in rail pads exposed to the railway
environment [
7
]. However, no specific study has been conducted to quantitatively measure
the magnitude of these effects on real components subjected to conditions representative
of the railway environment. The mechanical deterioration caused by repetitive loads was
examined by applying load blocks of 250,000, 750,000, and 2,000,000 cycles. Chemical
deterioration was also investigated by exposing the rail pads to ultraviolet radiation and
to the presence of hydrocarbons, respectively. In both cases, exposure times of 100 and
500 h were utilized. Overall, the study encompasses 21 distinct experimental conditions
(three types of pads and seven deterioration conditions), resulting in a comprehensive
experimental scope consisting of 864 tests.
The remainder of this paper is structured as follows: Section 2describes the rail
pads that were characterized in this study. Section 3outlines the experimental methods
employed to obtain the stiffness of the pads, along with the experimental conditions that
were utilized. The obtained results are presented and discussed in Section 4. Finally,
Section 5provides a summary of the findings and presents the conclusions that can be
drawn from this study.
2. Materials
For this study, three distinct types of commercially available railway pads provided
by the company PANDROL were utilized, as described in Figure 2. Hereafter, these rail
pads will be referred to as pad A, pad B, and pad C, respectively.
Appl. Sci. 2025, 15, x FOR PEER REVIEW 4 of 17
research identifying specific degradation mechanisms in rail pads exposed to the railway
environment [7]. However, no specific study has been conducted to quantitatively meas-
ure the magnitude of these effects on real components subjected to conditions representa-
tive of the railway environment. The mechanical deterioration caused by repetitive loads
was examined by applying load blocks of 250,000, 750,000, and 2,000,000 cycles. Chemical
deterioration was also investigated by exposing the rail pads to ultraviolet radiation and
to the presence of hydrocarbons, respectively. In both cases, exposure times of 100 and
500 h were utilized. Overall, the study encompasses 21 distinct experimental conditions
(three types of pads and seven deterioration conditions), resulting in a comprehensive
experimental scope consisting of 864 tests.
The remainder of this paper is structured as follows: Section 2 describes the rail pads
that were characterized in this study. Section 3 outlines the experimental methods em-
ployed to obtain the stiffness of the pads, along with the experimental conditions that were
utilized. The obtained results are presented and discussed in Section 4. Finally, Section 5
provides a summary of the findings and presents the conclusions that can be drawn from
this study.
2. Materials
For this study, three distinct types of commercially available railway pads provided
by the company PANDROL were utilized, as described in Figure 2. Hereafter, these rail
pads will be referred to as pad A, pad B, and pad C, respectively.
• Pad A: Rail pad fabricated from EPDM (Ethylene Propylene Diene Methylene), fea-
turing circular protrusions with a height of 2.5 mm and a total thickness of 11 mm.
• Pad B: Rail pad fabricated from EPDM (Ethylene Propylene Diene Methylene), fea-
turing circular protrusions with a height of 1.5 mm and a total thickness of 11 mm.
• Pad C: Rail pad fabricated from EVA (Ethylene Vinyl Acetate), featuring circular pro-
trusions with a height of 2 mm and a total thickness of 10 mm.
Rail pad A
Rail pad B
Rail pad C
Figure 2. Images depicting the various types of rail pads analyzed in this study.
3. Methods
The objective of this work is to experimentally quantify the influence of mechanical
and environmental factors on the performance of EPDM and EVA rail pads. For this pur-
pose, 21 combinations of aging scenarios were analyzed, and the results were compared
with tests on identical rail pads used as a reference, which were not subjected to any deg-
radation conditions. Each scenario corresponds to the application of one of the seven types
of experimental degradation conditions to each pad model, adding one more for the
Figure 2. Images depicting the various types of rail pads analyzed in this study.
•
Pad A: Rail pad fabricated from EPDM (Ethylene Propylene Diene Methylene), featur-
ing circular protrusions with a height of 2.5 mm and a total thickness of 11 mm.
•
Pad B: Rail pad fabricated from EPDM (Ethylene Propylene Diene Methylene), featur-
ing circular protrusions with a height of 1.5 mm and a total thickness of 11 mm.
•
Pad C: Rail pad fabricated from EVA (Ethylene Vinyl Acetate), featuring circular
protrusions with a height of 2 mm and a total thickness of 10 mm.
3. Methods
The objective of this work is to experimentally quantify the influence of mechanical
and environmental factors on the performance of EPDM and EVA rail pads. For this
purpose, 21 combinations of aging scenarios were analyzed, and the results were compared
with tests on identical rail pads used as a reference, which were not subjected to any
Appl. Sci. 2025,15, 4394 5 of 16
degradation conditions. Each scenario corresponds to the application of one of the seven
types of experimental degradation conditions to each pad model, adding one more for the
reference condition. The 864 dynamic stiffness tests conducted resulted from combining all
the main test parameters, which are three load amplitude values (15.5, 21, and 31.5 kN),
four tow load values (1, 9, 18, and 25 kN), and three test frequencies (5, 10, and 20 Hz) for
each pad model, collected in Table 1. All these tests were carried out in accordance with
the European Standards UNE-EN 13146-9 [18] and UNE-EN 13481-2 [19].
Table 1. Distribution of experimental test combinations.
Variable Values Number of Combinations
Load Amplitude (kN) 15.5, 21.0, 31.5 3
Tow Load (kN) 1, 9, 18, 25 4
Frequency (Hz) 5, 10, 20 3
Total per condition 3 ×4×3 = 36
Total Tests 864 (36 tests ×24 conditions)
3.1. Degradation Conditions
The three types of pads were subjected either to a mechanical or a chemical degradation
process, as described next. Table 2summarizes the different conditions imposed.
Table 2. Types of degradation imposed on the pads.
Type of Degradation Condition Code
Reference Conditions R
Mechanical
250,000 cycles M1
750,000 cycles M2
2,000,000 cycles M3
Environmental
Exposure to UV light 100 h UV1
500 h UV2
Exposure to hydrocarbons 100 h HC1
500 h HC2
The mechanical degradation resulting from the repeated passage of trains was simu-
lated experimentally by subjecting the rail pads to cyclic loading under typical high-speed
conditions. For this purpose, a load amplitude of 31.5 kN, a preload of 18 kN, and a fre-
quency of 5 Hz were applied. The extent of damage was regulated by varying the number
of cycles applied, considering three levels: 250,000, 750,000, and 2,000,000 cycles.
Chemical degradation is intended to replicate the impact of various agents to which
the pads may be exposed throughout their service life on the track. Two aging conditions
were considered: exposure to ultraviolet (UV) light and exposure to hydrocarbons. For
both cases, the effects were assessed after 100 and 500 h of exposure.
The UV exposure tests were conducted using an Atlas UV2000 (manufacturer: Atlas
MTS, made in USA) testing device. Samples were subjected to sequential 6 h cycles,
each consisting of 5 h of UV radiation exposure at 50
◦
C with an energy intensity of
0.77 W/m
2·
nm, provided by UV-430 fluorescent lamps, followed by a 1 h water spray
period. For hydrocarbon resistance tests, rail pads were immersed in NUTO H-46 oil,
commonly utilized for lubricating industrial equipment and hydraulic systems in the
railway sector. Samples were kept submerged for durations of either 100 or 500 h.
Appl. Sci. 2025,15, 4394 6 of 16
3.2. Mechanical Characterization: Dynamic Stiffness Tests
Stiffness tests were conducted using a universal servo-hydraulic testing machine
equipped with a
±
100 kN capacity load cell, as can be shown in Figure 3. The deformation
of the rail pads was recorded using four Linear Variable Differential Transformers (LVDTs)
mounted on a metallic base replicating the sleeper’s geometry. The loads were applied to
the rail pads through a UIC60 rail sample, which was connected to the testing machine via
a ball joint to ensure the vertical alignment of the applied load. Throughout the tests, both
the vertical load and the rail pad deformation were measured, with the latter calculated as
the average of the four LVDTs.
Appl. Sci. 2025, 15, x FOR PEER REVIEW 6 of 17
applied to the rail pads through a UIC60 rail sample, which was connected to the testing
machine via a ball joint to ensure the vertical alignment of the applied load. Throughout
the tests, both the vertical load and the rail pad deformation were measured, with the
laer calculated as the average of the four LVDTs.
Figure 3. Image of the stiffness test conducted on a testing machine.
According to standards UNE-EN 13481-2 [19] and UNE-EN 13146-9 [18], dynamic
stiffness was determined by subjecting the rail pad to 1000 sinusoidal load cycles. An ex-
ample of a force-displacement curve required to evaluate the dynamic stiffness of the pads
is presented in Figure 4.
The dynamic stiffness of the rail pads was calculated as the ratio of the average load
range to the average displacement range over the final 100 cycles of each test, as expressed
in Equation (1):
𝑘 =
(1)
The standard establishes a reference frequency of 5 Hz but allows testing at 10 and
20 Hz for specific cases, so tests were conducted at these three frequencies. Regarding load
amplitude, the standard provides different values based on the track category; therefore,
all specified values (15.5, 21.0, and 31.5 kN) were considered. For the minimum test load,
the standard specifies a value of 18 kN, representing the fastening’s tightening. However,
since fastening tightening varies significantly, four different minimum load values were
selected: 1 kN (simulating a broken fastening), 9 kN (loosened fastening), 18 kN (reference
condition), and 25 kN (overtightened fastening).
Figure 3. Image of the stiffness test conducted on a testing machine.
According to standards UNE-EN 13481-2 [
19
] and UNE-EN 13146-9 [
18
], dynamic
stiffness was determined by subjecting the rail pad to 1000 sinusoidal load cycles. An
example of a force-displacement curve required to evaluate the dynamic stiffness of the
pads is presented in Figure 4.
The dynamic stiffness of the rail pads was calculated as the ratio of the average load
range to the average displacement range over the final 100 cycles of each test, as expressed
in Equation (1):
kdyn =
Fmax −Fmin
Dmax −Dmin
(1)
The standard establishes a reference frequency of 5 Hz but allows testing at 10 and
20 Hz for specific cases, so tests were conducted at these three frequencies. Regarding load
amplitude, the standard provides different values based on the track category; therefore,
all specified values (15.5, 21.0, and 31.5 kN) were considered. For the minimum test load,
the standard specifies a value of 18 kN, representing the fastening’s tightening. However,
since fastening tightening varies significantly, four different minimum load values were
selected: 1 kN (simulating a broken fastening), 9 kN (loosened fastening), 18 kN (reference
condition), and 25 kN (overtightened fastening).
Appl. Sci. 2025,15, 4394 7 of 16
Appl. Sci. 2025, 15, x FOR PEER REVIEW 7 of 17
Figure 4. Graph showing the force–displacement curve for a dynamic test.
4. Results
Figure 5 shows the differences in dynamic stiffness obtained when evaluating the
three reference pads under the standard parameters for high-speed railways (5 Hz fre-
quency, 31.5 kN amplitude, and 18 kN toe load). These parameters will serve as a refer-
ence for the comparative analysis of the dynamic stiffness curves recorded under the dif-
ferent experimental conditions, which are presented in subsequent graphs for each of the
pads analyzed.
Figure 5. Example of dynamic stiffness graphs of reference rail pads under standard high-speed rail
conditions.
Figure 6 provides an overview of the behavior, presenting box plots that illustrate
the distribution of dynamic stiffness for each pad and experimental condition. It can be
observed that pad A exhibits the lowest stiffness and variability compared to the other
two types. The average values range from 43 kN in condition HC1 to 58 kN in condition
M3, while the reference pad achieves an average of 49 kN. The standard deviation ranges
between 20 and 30 kN.
Pad B presents a mean stiffness value ranging from 101 kN in the UV1 condition to
146 kN in the M3 condition, with a standard deviation that increases significantly,
Figure 4. Graph showing the force–displacement curve for a dynamic test.
4. Results
Figure 5shows the differences in dynamic stiffness obtained when evaluating the
three reference pads under the standard parameters for high-speed railways (5 Hz fre-
quency, 31.5 kN amplitude, and 18 kN toe load). These parameters will serve as a reference
for the comparative analysis of the dynamic stiffness curves recorded under the differ-
ent experimental conditions, which are presented in subsequent graphs for each of the
pads analyzed.
Appl. Sci. 2025, 15, x FOR PEER REVIEW 7 of 17
Figure 4. Graph showing the force–displacement curve for a dynamic test.
4. Results
Figure 5 shows the differences in dynamic stiffness obtained when evaluating the
three reference pads under the standard parameters for high-speed railways (5 Hz fre-
quency, 31.5 kN amplitude, and 18 kN toe load). These parameters will serve as a refer-
ence for the comparative analysis of the dynamic stiffness curves recorded under the dif-
ferent experimental conditions, which are presented in subsequent graphs for each of the
pads analyzed.
Figure 5. Example of dynamic stiffness graphs of reference rail pads under standard high-speed rail
conditions.
Figure 6 provides an overview of the behavior, presenting box plots that illustrate
the distribution of dynamic stiffness for each pad and experimental condition. It can be
observed that pad A exhibits the lowest stiffness and variability compared to the other
two types. The average values range from 43 kN in condition HC1 to 58 kN in condition
M3, while the reference pad achieves an average of 49 kN. The standard deviation ranges
between 20 and 30 kN.
Pad B presents a mean stiffness value ranging from 101 kN in the UV1 condition to
146 kN in the M3 condition, with a standard deviation that increases significantly,
Figure 5. Example of dynamic stiffness graphs of reference rail pads under standard high-speed
rail conditions.
Figure 6provides an overview of the behavior, presenting box plots that illustrate
the distribution of dynamic stiffness for each pad and experimental condition. It can be
observed that pad A exhibits the lowest stiffness and variability compared to the other
two types. The average values range from 43 kN in condition HC1 to 58 kN in condition
M3, while the reference pad achieves an average of 49 kN. The standard deviation ranges
between 20 and 30 kN.
Pad B presents a mean stiffness value ranging from 101 kN in the UV1 condition
to 146 kN in the M3 condition, with a standard deviation that increases significantly,
Appl. Sci. 2025,15, 4394 8 of 16
fluctuating between 66 kN and 122 kN; thus, this material exhibits the greatest variability
among the three pads analyzed.
Appl. Sci. 2025, 15, x FOR PEER REVIEW 8 of 17
fluctuating between 66 kN and 122 kN; thus, this material exhibits the greatest variability
among the three pads analyzed.
Regarding pad C, a high dispersion in its values is also observed. It is also the stiffest
of the three pads analyzed, with stiffness values ranging from 348 kN in condition UV1 to
506 kN in condition M3, while the standard deviation varies between 83 kN and 125 kN.
Figure 6. Boxplots distribution of dynamic stiffness for each rail pad and testing condition.
Figure 7 is presented to illustrate the mean relative stiffness of the different degrada-
tion agents compared to the reference pad. This behavior is analyzed in greater detail in
Sections 4.1, 4.2, and 4.3, where the effects of each condition will be examined individu-
ally.
Figure 6. Boxplots distribution of dynamic stiffness for each rail pad and testing condition.
Regarding pad C, a high dispersion in its values is also observed. It is also the stiffest
of the three pads analyzed, with stiffness values ranging from 348 kN in condition UV1 to
506 kN in condition M3, while the standard deviation varies between 83 kN and 125 kN.
Figure 7is presented to illustrate the mean relative stiffness of the different degradation
agents compared to the reference pad. This behavior is analyzed in greater detail in
Sections 4.1–4.3, where the effects of each condition will be examined individually.
Appl. Sci. 2025, 15, x FOR PEER REVIEW 9 of 17
Figure 7. Average relative impact of degradation agents compared to the reference rail pad.
4.1. Type A Pads
Dynamic stiffness curves obtained from tests on the various conditions of pad A,
conducted under the aforementioned reference parameters, are depicted in Figure 8. The
results indicate that mechanical degradation increases the stiffness of the pads as the num-
ber of cycles increases, while exposure to UV radiation causes a decrease in stiffness. In
the case of hydrocarbons, a mixed effect is observed: the condition with higher exposure
increases stiffness, whereas that with lower exposure reduces it.
Figure 8. Dynamic stiffness curves obtained from tests under different conditions for pad A.
The boxplot diagram presented in Figure 9 illustrates the variation in dynamic stiff-
ness obtained for the different combinations of test parameters compared to the value ob-
tained in the reference pad, allowing visualization of the distribution of stiffness variation
Figure 7. Average relative impact of degradation agents compared to the reference rail pad.
Appl. Sci. 2025,15, 4394 9 of 16
4.1. Type A Pads
Dynamic stiffness curves obtained from tests on the various conditions of pad A,
conducted under the aforementioned reference parameters, are depicted in Figure 8. The
results indicate that mechanical degradation increases the stiffness of the pads as the
number of cycles increases, while exposure to UV radiation causes a decrease in stiffness.
In the case of hydrocarbons, a mixed effect is observed: the condition with higher exposure
increases stiffness, whereas that with lower exposure reduces it.
Appl. Sci. 2025, 15, x FOR PEER REVIEW 9 of 17
Figure 7. Average relative impact of degradation agents compared to the reference rail pad.
4.1. Type A Pads
Dynamic stiffness curves obtained from tests on the various conditions of pad A,
conducted under the aforementioned reference parameters, are depicted in Figure 8. The
results indicate that mechanical degradation increases the stiffness of the pads as the num-
ber of cycles increases, while exposure to UV radiation causes a decrease in stiffness. In
the case of hydrocarbons, a mixed effect is observed: the condition with higher exposure
increases stiffness, whereas that with lower exposure reduces it.
Figure 8. Dynamic stiffness curves obtained from tests under different conditions for pad A.
The boxplot diagram presented in Figure 9 illustrates the variation in dynamic stiff-
ness obtained for the different combinations of test parameters compared to the value ob-
tained in the reference pad, allowing visualization of the distribution of stiffness variation
Figure 8. Dynamic stiffness curves obtained from tests under different conditions for pad A.
The boxplot diagram presented in Figure 9illustrates the variation in dynamic
stiffness obtained for the different combinations of test parameters compared to the
value obtained in the reference pad, allowing visualization of the distribution of stiffness
variation for each case and condition. It is observed that the influence of mechanical
degradation progressively intensifies, with mean increases in dynamic stiffness of 2%,
3.5%, and 17% for M1, M2, and M3, respectively, while exposure to UV light reduces
dynamic stiffness by 5.5% and 5% for UV1 and UV2, respectively, thereby increasing the
dispersion relative to the effects of mechanical degradation. On the other hand, exposure
to hydrocarbons results in an average variation in dynamic stiffness of
−
11.5% for HC1
and
−
3% for HC2; in the latter case, a high dispersion in the data is observed, with some
measurements showing increases in dynamic stiffness of over 20%, while others exhibit
decreases of more than 20%.
To explain the greater dispersion observed in the chemical degradation results, espe-
cially in cases of higher exposure, a scatter diagram is presented in Figure 10 that correlates
the rail pad stiffness with the tow load and the maximum load applied during dynamic
tests. The stiffness variation is represented using a color scale. This diagram reveals that in
the case of UV radiation exposure, an increase in stiffness is observed with the application
of low loads, whereas as the load increases, the stiffness decreases. Moreover, this behavior
intensifies as exposure to UV light increases.
A scatterplot similar to Figure 10, used to analyze the exposure of the same rail
pads to hydrocarbons, has already been presented and studied by the same authors in a
previous publication [20].
Appl. Sci. 2025,15, 4394 10 of 16
Appl. Sci. 2025, 15, x FOR PEER REVIEW 10 of 17
for each case and condition. It is observed that the influence of mechanical degradation
progressively intensifies, with mean increases in dynamic stiffness of 2%, 3.5%, and 17%
for M1, M2, and M3, respectively, while exposure to UV light reduces dynamic stiffness
by 5.5% and 5% for UV1 and UV2, respectively, thereby increasing the dispersion relative
to the effects of mechanical degradation. On the other hand, exposure to hydrocarbons
results in an average variation in dynamic stiffness of −11.5% for HC1 and −3% for HC2;
in the laer case, a high dispersion in the data is observed, with some measurements
showing increases in dynamic stiffness of over 20%, while others exhibit decreases of more
than 20%.
Figure 9. Boxplots showing the effect of each type of aging stiffness for rail pad A.
To explain the greater dispersion observed in the chemical degradation results, espe-
cially in cases of higher exposure, a scaer diagram is presented in Figure 10 that corre-
lates the rail pad stiffness with the tow load and the maximum load applied during dy-
namic tests. The stiffness variation is represented using a color scale. This diagram reveals
that in the case of UV radiation exposure, an increase in stiffness is observed with the
application of low loads, whereas as the load increases, the stiffness decreases. Moreover,
this behavior intensifies as exposure to UV light increases.
Figure 9. Boxplots showing the effect of each type of aging stiffness for rail pad A.
Appl. Sci. 2025, 15, x FOR PEER REVIEW 10 of 17
for each case and condition. It is observed that the influence of mechanical degradation
progressively intensifies, with mean increases in dynamic stiffness of 2%, 3.5%, and 17%
for M1, M2, and M3, respectively, while exposure to UV light reduces dynamic stiffness
by 5.5% and 5% for UV1 and UV2, respectively, thereby increasing the dispersion relative
to the effects of mechanical degradation. On the other hand, exposure to hydrocarbons
results in an average variation in dynamic stiffness of −11.5% for HC1 and −3% for HC2;
in the laer case, a high dispersion in the data is observed, with some measurements
showing increases in dynamic stiffness of over 20%, while others exhibit decreases of more
than 20%.
Figure 9. Boxplots showing the effect of each type of aging stiffness for rail pad A.
To explain the greater dispersion observed in the chemical degradation results, espe-
cially in cases of higher exposure, a scaer diagram is presented in Figure 10 that corre-
lates the rail pad stiffness with the tow load and the maximum load applied during dy-
namic tests. The stiffness variation is represented using a color scale. This diagram reveals
that in the case of UV radiation exposure, an increase in stiffness is observed with the
application of low loads, whereas as the load increases, the stiffness decreases. Moreover,
this behavior intensifies as exposure to UV light increases.
Figure 10. Scatterplot correlating rail pad stiffness with tow load and maximum applied load for UV
exposition in pad A.
4.2. Type B Pads
Dynamic stiffness curves experimentally obtained for the different conditions of pad B
reveal that, in the case of mechanical degradation, the behavior is similar to that of pad A,
with a constant increase in stiffness as exposure increases. UV exposure results in a decrease
in stiffness, although the effect is more pronounced under lower exposure conditions, while
exposure to hydrocarbons tends to reduce stiffness as exposure increases, as illustrated
in Figure 11.
The variation in dynamic stiffness of pad B, as illustrated in the plots in Figure 12,
is similar to that observed in pad A, showing average increases of 5.5%, 9%, and 15% for
the M1, M2, and M3 conditions, respectively. Exposure to UV light reduces stiffness by
11.5% and 7.5% for UV1 and UV2, respectively. With regard to hydrocarbon exposure, the
average variation indicates a slight reduction in dynamic stiffness, close to
−
1%, affecting
both exposure levels similarly. It is worth noting that although the dispersion of results is
comparable for conditions HC1 and HC2, lower variability is observed in HC2, whose data
cluster into two trends: one that increases stiffness by between 10% and 20% and another
that reduces it by a similar ranger.
Figure 13 provides evidence of behavior similar to that observed in pad A regarding
UV exposure, although the magnitude of the effects is greater for this pad.
Appl. Sci. 2025,15, 4394 11 of 16
Appl. Sci. 2025, 15, x FOR PEER REVIEW 11 of 17
Figure 10. Scaerplot correlating rail pad stiffness with tow load and maximum applied load for
UV exposition in pad A.
A scaerplot similar to Figure 10, used to analyze the exposure of the same rail pads
to hydrocarbons, has already been presented and studied by the same authors in a previ-
ous publication [20].
4.2. Type B Pads
Dynamic stiffness curves experimentally obtained for the different conditions of pad
B reveal that, in the case of mechanical degradation, the behavior is similar to that of pad
A, with a constant increase in stiffness as exposure increases. UV exposure results in a
decrease in stiffness, although the effect is more pronounced under lower exposure con-
ditions, while exposure to hydrocarbons tends to reduce stiffness as exposure increases,
as illustrated in Figure 11.
Figure 11. Dynamic stiffness curves obtained from tests under different conditions for pad B.
The variation in dynamic stiffness of pad B, as illustrated in the plots in Figure 12,
is similar to that observed in pad A, showing average increases of 5.5%, 9%, and 15% for
the M1, M2, and M3 conditions, respectively. Exposure to UV light reduces stiffness by
11.5% and 7.5% for UV1 and UV2, respectively. With regard to hydrocarbon exposure, the
average variation indicates a slight reduction in dynamic stiffness, close to −1%, affecting
both exposure levels similarly. It is worth noting that although the dispersion of results is
comparable for conditions HC1 and HC2, lower variability is observed in HC2, whose
data cluster into two trends: one that increases stiffness by between 10% and 20% and
another that reduces it by a similar ranger.
Figure 11. Dynamic stiffness curves obtained from tests under different conditions for pad B.
Appl. Sci. 2025, 15, x FOR PEER REVIEW 12 of 17
Figure 12. Boxplots showing the effect of each type of aging stiffness for rail pad B.
Figure 13 provides evidence of behavior similar to that observed in pad A regarding
UV exposure, although the magnitude of the effects is greater for this pad.
Figure 13. Scaerplot correlating rail pad stiffness with tow load and maximum applied load for
UV exposition in pad B.
4.3. Type C Pads
Dynamic stiffness curves obtained from tests for the different conditions of pad C
reveal an increase in stiffness with escalating mechanical degradation, although no effect
is observed under low exposure levels. UV light exposure exhibits a mixed effect, decreas-
ing stiffness under low exposure conditions while increasing it under more intense expo-
sures, whereas the influence of hydrocarbon exposure is relatively mild, causing a slight
reduction in stiffness as exposure increases, as shown in Figure 14.
Figure 12. Boxplots showing the effect of each type of aging stiffness for rail pad B.
Appl. Sci. 2025, 15, x FOR PEER REVIEW 12 of 17
Figure 12. Boxplots showing the effect of each type of aging stiffness for rail pad B.
Figure 13 provides evidence of behavior similar to that observed in pad A regarding
UV exposure, although the magnitude of the effects is greater for this pad.
Figure 13. Scaerplot correlating rail pad stiffness with tow load and maximum applied load for
UV exposition in pad B.
4.3. Type C Pads
Dynamic stiffness curves obtained from tests for the different conditions of pad C
reveal an increase in stiffness with escalating mechanical degradation, although no effect
is observed under low exposure levels. UV light exposure exhibits a mixed effect, decreas-
ing stiffness under low exposure conditions while increasing it under more intense expo-
sures, whereas the influence of hydrocarbon exposure is relatively mild, causing a slight
reduction in stiffness as exposure increases, as shown in Figure 14.
Figure 13. Scatterplot correlating rail pad stiffness with tow load and maximum applied load for UV
exposition in pad B.
Appl. Sci. 2025,15, 4394 12 of 16
4.3. Type C Pads
Dynamic stiffness curves obtained from tests for the different conditions of pad C
reveal an increase in stiffness with escalating mechanical degradation, although no effect is
observed under low exposure levels. UV light exposure exhibits a mixed effect, decreasing
stiffness under low exposure conditions while increasing it under more intense exposures,
whereas the influence of hydrocarbon exposure is relatively mild, causing a slight reduction
in stiffness as exposure increases, as shown in Figure 14.
Appl. Sci. 2025, 15, x FOR PEER REVIEW 13 of 17
Figure 14. Dynamic stiffness curves obtained from tests under different conditions for pad C.
Pad C shows, through the boxplots presented in Figure 15, an increase in stiffness
under the mechanical degradation conditions M1, M2, and M3 of 2.5%, 22%, and 27%,
respectively. Regarding exposure to UV light, a decrease of 13% is observed for UV1 and
an increase of 9.5% for UV2. On the other hand, exposure to hydrocarbons reduces dy-
namic stiffness by 5% under HC1, whereas in HC2, a 5% increase is recorded.
Figure 15. Boxplots showing the effect of each type of aging stiffness for rail pad C.
Figure 16 shows that exposure to UV light produces a mixed effect on this pad; it
reduces the stiffness in conditions of low exposure and increases it in those of higher
Figure 14. Dynamic stiffness curves obtained from tests under different conditions for pad C.
Pad C shows, through the boxplots presented in Figure 15, an increase in stiffness
under the mechanical degradation conditions M1, M2, and M3 of 2.5%, 22%, and 27%,
respectively. Regarding exposure to UV light, a decrease of 13% is observed for UV1 and an
increase of 9.5% for UV2. On the other hand, exposure to hydrocarbons reduces dynamic
stiffness by 5% under HC1, whereas in HC2, a 5% increase is recorded.
Appl. Sci. 2025, 15, x FOR PEER REVIEW 13 of 17
Figure 14. Dynamic stiffness curves obtained from tests under different conditions for pad C.
Pad C shows, through the boxplots presented in Figure 15, an increase in stiffness
under the mechanical degradation conditions M1, M2, and M3 of 2.5%, 22%, and 27%,
respectively. Regarding exposure to UV light, a decrease of 13% is observed for UV1 and
an increase of 9.5% for UV2. On the other hand, exposure to hydrocarbons reduces dy-
namic stiffness by 5% under HC1, whereas in HC2, a 5% increase is recorded.
Figure 15. Boxplots showing the effect of each type of aging stiffness for rail pad C.
Figure 16 shows that exposure to UV light produces a mixed effect on this pad; it
reduces the stiffness in conditions of low exposure and increases it in those of higher
Figure 15. Boxplots showing the effect of each type of aging stiffness for rail pad C.
Appl. Sci. 2025,15, 4394 13 of 16
Figure 16 shows that exposure to UV light produces a mixed effect on this pad; it
reduces the stiffness in conditions of low exposure and increases it in those of higher
exposure. In addition, it is observed that, for small load values, the stiffness is higher in
both cases.
Appl. Sci. 2025, 15, x FOR PEER REVIEW 14 of 17
exposure. In addition, it is observed that, for small load values, the stiffness is higher in
both cases.
Figure 16. Scaerplot correlating rail pad stiffness with tow load and maximum applied load for
UV exposition in pad C.
5. Discussion
This study has analyzed the influence of the main mechanical and chemical aging
processes to which rail pads are subjected during their service life, demonstrating their
effect on the dynamic stiffness of three different types of pads manufactured with EPDM
and EVA, materials commonly used by rail pad manufacturers. To achieve this, an exper-
imental program was carried out that included 864 dynamic stiffness tests under different
degradation scenarios. The results allow for a more detailed understanding of how deg-
radation mechanisms affect the dynamic stiffness of the pads under various aging condi-
tions, which is fundamental for the proper design and maintenance of the railway super-
structure.
The importance of the degradation of polymers used in the railway sector has been
demonstrated by Ito et al. [12], who showed in their article how the mechanical properties
of rail pads are affected by exposure to temperature, ultraviolet (UV) radiation, and deg-
radation by chemical, biological, and mechanical agents. Mechanical degradation progres-
sively increases the dynamic stiffness in all types of pads, with the most notable increase
observed in the EVA pad (pad C in this study), which reached a 27% increase in the
2,000,000-cycle scenario. These results agree with those obtained by Carrascal et al. [21],
who observed significant increases in both static and dynamic stiffness for EPDM rail pads
subjected to mechanical fatigue aging. One possible reason for these results is that the
loads experienced by the rail pads may lead to a reduction or deformation of the protru-
sion area, resulting in an increase in stiffness. This hypothesis is consistent with the find-
ings of Sol-Sánchez et al. [22], who observed that the permanent deformation experienced
by rail pads due to fatigue and repeated loading causes a significant increase in their dy-
namic stiffness. Specifically, they found that pads exhibiting greater permanent defor-
mation after repeated loading cycles also showed greater increases in stiffness. UV expo-
sure has different effects depending on the material and the pad, although a general trend
is observed in all cases: for low exposures, the stiffness is lower than for high exposures.
This is most evident in the case of pad C, manufactured in EVA, where a 13% decrease in
dynamic stiffness is observed under the lowest exposure condition and a 9.5% increase
under the highest exposure. The effect of UV exposure on polymers has been studied by
other authors [12,23], such as Jin et al. [23], who demonstrated that exposure to UV light
causes modifications in the structure of the EVA polymer, affecting its mechanical prop-
erties so that short exposures improve mechanical behavior while prolonged exposures
Figure 16. Scatterplot correlating rail pad stiffness with tow load and maximum applied load for UV
exposition in pad C.
5. Discussion
This study has analyzed the influence of the main mechanical and chemical aging pro-
cesses to which rail pads are subjected during their service life, demonstrating their effect
on the dynamic stiffness of three different types of pads manufactured with EPDM and
EVA, materials commonly used by rail pad manufacturers. To achieve this, an experimental
program was carried out that included 864 dynamic stiffness tests under different degrada-
tion scenarios. The results allow for a more detailed understanding of how degradation
mechanisms affect the dynamic stiffness of the pads under various aging conditions, which
is fundamental for the proper design and maintenance of the railway superstructure.
The importance of the degradation of polymers used in the railway sector has been
demonstrated by Ito et al. [
12
], who showed in their article how the mechanical properties of
rail pads are affected by exposure to temperature, ultraviolet (UV) radiation, and degrada-
tion by chemical, biological, and mechanical agents. Mechanical degradation progressively
increases the dynamic stiffness in all types of pads, with the most notable increase observed
in the EVA pad (pad C in this study), which reached a 27% increase in the 2,000,000-cycle
scenario. These results agree with those obtained by Carrascal et al. [
21
], who observed
significant increases in both static and dynamic stiffness for EPDM rail pads subjected to
mechanical fatigue aging. One possible reason for these results is that the loads experienced
by the rail pads may lead to a reduction or deformation of the protrusion area, resulting
in an increase in stiffness. This hypothesis is consistent with the findings of Sol-Sánchez
et al. [
22
], who observed that the permanent deformation experienced by rail pads due
to fatigue and repeated loading causes a significant increase in their dynamic stiffness.
Specifically, they found that pads exhibiting greater permanent deformation after repeated
loading cycles also showed greater increases in stiffness. UV exposure has different effects
depending on the material and the pad, although a general trend is observed in all cases:
for low exposures, the stiffness is lower than for high exposures. This is most evident
in the case of pad C, manufactured in EVA, where a 13% decrease in dynamic stiffness
is observed under the lowest exposure condition and a 9.5% increase under the highest
exposure. The effect of UV exposure on polymers has been studied by other authors [
12
,
23
],
such as Jin et al. [
23
], who demonstrated that exposure to UV light causes modifications in
Appl. Sci. 2025,15, 4394 14 of 16
the structure of the EVA polymer, affecting its mechanical properties so that short exposures
improve mechanical behavior while prolonged exposures deteriorate it. These findings
differ from those obtained in the present study, where it is observed that, in the case of rail
pads, geometry plays a crucial role, as noted by other studies [
10
,
24
]. It is likely that, under
low exposure conditions, material degradation reduces the load-bearing capacity of the
protrusions, leading to a decrease in dynamic stiffness, while for higher exposure condi-
tions, their load-bearing capacity is exhausted, which would induce an increase in stiffness,
as seen in Figure 16 Furthermore, it should be noted that the evaluation of mechanical
properties by Jin et al. [
23
] is carried out through tensile tests, whereas in the present study,
dynamic stiffness was obtained through compression fatigue tests at different frequencies.
Hydrocarbon exposure also produces different effects depending on the material and the
pad. In the case of pad A, a reduction in stiffness is observed under both conditions, with an
11.5% loss at the lower exposure level and a partial recovery, reaching only a 3% decrease
at the higher exposure level. On the other hand, a slight 5% increase was recorded for pad
C under prolonged exposure. Our conclusions agree with those previously obtained by
Sainz-Aja et al. [20].
In the case of exposure to hydrocarbons and UV radiation, the observed differences
in effects between rail pads and exposure levels could reasonably be attributed to the
substantial differences in how the polymer chains constituting EPDM and EVA respond to
degradation processes. Thus, previous studies have already indicated the difference in the
deterioration caused by environmental agents between different polymers [
7
]. Specifically,
the distinct chemical structure of each material influences its degradation mechanisms,
resulting in different behaviors under identical environmental conditions. In this way, the
variations in dynamic stiffness observed in this study reflect the underlying chemical and
structural characteristics unique to each polymer [
13
–
17
], affecting their performance and
durability in real operating scenarios.
Finally, although this study provides insights into the degradation mechanisms and
their effects on the dynamic stiffness of rail pads made of EPDM and EVA, there are several
important limitations that must be acknowledged. Firstly, while the experimental tests
consider representative conditions of the railway environment, the effects of mechanical
aging, ultraviolet radiation, and hydrocarbon exposure were evaluated separately without
analyzing the possible interactions or simultaneous combinations of these factors, which
may occur under real service conditions. Additionally, the study focuses exclusively on
two specific materials with specific geometries, implying that the results obtained within
this research may not be totally generalizable to other different components.
6. Conclusions
This study provides a detailed characterization of some of the main degradation
mechanisms affecting the service life and performance of rail pads, which are essential com-
ponents for ensuring the safety, comfort, and durability of railway infrastructure. Through
experimental tests carried out under conditions representative of the railway environment,
it has been established that the aging behavior significantly depends on both the material
used (EPDM or EVA) and the geometry of the component. Additionally, the study quanti-
tatively evaluates how specific variables, such as mechanical fatigue, ultraviolet radiation,
and exposure to hydrocarbons, influence the dynamic stiffness of these components. These
results allow for a more precise selection of rail pad materials and designs based on the
specific operational conditions they will face in service. Furthermore, the insights gained
provide the foundation for optimizing preventive maintenance strategies by enabling a
more accurate understanding of track stiffness, thus contributing to reduced operating
costs, minimized service disruptions, and extended component lifetimes. Future research
Appl. Sci. 2025,15, 4394 15 of 16
should address the combination of different degradation agents on the same rail pad, as
well as further explore the role that geometry plays in its deterioration.
In conclusion, this study has provided a detailed characterization of how mechanical
and chemical aging processes influence the dynamic stiffness of rail pads manufactured
with EPDM and EVA, materials widely used in the railway industry. The results show that
mechanical degradation progressively increases the dynamic stiffness of rail pads, with EVA
pads being the most affected, exhibiting stiffness increases of up to 27%. On the other hand,
exposure to UV radiation produced variable effects depending on exposure intensity and
pad geometry, initially decreasing stiffness at lower exposures and subsequently increasing
it at prolonged exposures. Additionally, hydrocarbon exposure also revealed different
behaviors depending on the material type. These findings highlight the importance of
considering aging mechanisms in the selection and proper maintenance of rail pads, laying
the groundwork for future research on combined effects and the influence of pad geometry.
Author Contributions: Conceptualization, I.R., J.A.S.-A., D.F. and I.C.; methodology, I.R., J.A.S.-A.,
I.C. and S.D.; software, I.R., J.A.S.-A. and D.F.; validation, I.R., J.A.S.-A. and D.F.; formal analysis,
I.R., J.A.S.-A. and D.F.; investigation, I.R.; data curation, I.R.; writing—original draft preparation,
I.R.; writing—review and editing, I.R., J.A.S.-A., D.F. and J.C.; visualization, I.R., J.A.S.-A. and D.F.;
supervision, J.A.S.-A. and D.F.; project administration, D.F., I.C., J.C. and S.D.; funding acquisition,
D.F., I.C., J.C. and S.D. All authors have read and agreed to the published version of the manuscript.
Funding: This publication is part of the R&D&I project PID2021-128031OB-I00 funded by MCIN
AEI/10.13039/501100011033 and by “ERDF A way of making Europe”.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data will be made available upon request.
Acknowledgments: This work was supported by PANDROL who provided the rail pads for the
experimental study.
Conflicts of Interest: The authors declare that they have no known competing financial interests or
personal relationships that could have appeared to influence the work reported in this paper.
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