# A Review of Railway Track Laboratory Tests with Various Scales for Better Decision-Making about More Efficient Apparatus Using TOPSIS Analysis

Authors: Vahid Najafi Moghaddam Gilani; Mohammad Habibzadeh; Seyed Mohsen Hosseinian; Reza Salehfard
Journal: Advances in Civil Engineering
 (2022)

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## Abstract

Improving the performance of railways depends on increasing their efficiency and the speed of locomotives, which is provided by improving the infrastructure of railways. Therefore, in various studies, researchers seek to modify ballast materials, sleepers and rails, or to study various geometric and environmental conditions in order to reduce the existing weaknesses of railways. Therefore, in order to evaluate the above conditions, various tests should be performed on railways. Some of the tests take place in the field when the train is passing. However, these tests have various limitations and endanger the safety of researchers and are associated with difficulties. Therefore, many tests are performed using various apparatuses on a laboratory scale. In this study, by reviewing different apparatuses used by researchers to simulate loading on the railway, they were compared and by examining different criteria such as loading type, different loading directions, scale of apparatuses, static and dynamic simulated loads and other environmental and geometric conditions, the best apparatuses on a laboratory scale are recommended for simulating vertical, longitudinal and lateral loads and measuring noise and vibration. By the use of TOPSIS analysis on 16 introduced apparatuses from reviewing different studies, it was found that the scaled railway track model had the first rank among different apparatuses and by modifying the weaknesses of this apparatus, it can be turned into the most comprehensive laboratory apparatus to simulate the loading of railway tracks with a scale of 1 : 5. Among the introduced apparatuses, wheel on roller test rig and roller rig had the second and third ranks, respectively, the apparatuses of which usually have a scale of 1 : 5 and deal with the interaction between the rail and the wheel and the impact of the vehicle on the railway track.


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## Body

## 1. Introduction

Today, various modes of transportation are being developed, and in the meantime, the railway is developing due to its safety and high speed compared to the road systems. Therefore, various technologies are being added to this industry and researchers are always trying to reduce the weaknesses of railway tracks and add more strengths in different studies day by day. These studies have been conducted in various fields, most of which have examined the suspension systems and mechanics of wheels and rails and the improvement of various fleet systems and the reduction of pollution [1–3]. Other studies have examined the transportation and traffic of railways and how to provide the most optimal railway tracks [4] or schedule the movement of trains in the minimum waiting time for passengers [5]. But the most important part of the previous studies is the study of the mechanical behavior of railway tracks and structures under the influence of different dynamic and static forces [6], the studies of which are divided into three parts, including modeling and simulation of tracks and structures using various software [7–9], field tests on real railways at train crossings [10, 11] and tests at different scales in the laboratory space. Among these three categories, related studies conducted in the laboratory at different scales are more popular due to the following benefits:(i)
Increasing the safety and performance of researchers during the test compared to the field method, as well as an easier examination of various parameters affecting the railway tracks(ii)
Ability to validate laboratory data as well as easier preparation of information for simulations and numerical modeling(iii)
Reducing the cost of construction and maintenance of laboratory apparatuses by considering different scales according to the intended objectives(iv)
Simultaneous study of different forces on the railway track and estimation of static and dynamic loadings on it and thus determining the lateral, longitudinal and creep resistances of the railway trackAll of the above make laboratory apparatuses unique with different scales, and many researchers have used these methods to measure lateral displacement, track settlement, permanent deformations, water level effect, ballast behavior under loading, fatigue life, settlement depth, lateral resistance of shoulder and space of the sleepers that depending on the objectives, different scales from 1 : 1 to 1 : 20 can be used. But the point to consider is which of these apparatuses is superior to the other and what scale of laboratory apparatuses provides the most objectives for static and dynamic loadings on the railway track. Certainly, each of these apparatuses is complementary to each other, and by summarizing and reviewing all the studies performed, it is necessary to present the laboratory-scale apparatuses that provide the highest objectives for evaluating railway tracks under different loads. Therefore, the main purpose of this review study is to provide a comparative analysis of laboratory methods at different scales in order to evaluate the resistance of railway tracks under different loads. The structure of this study is illustrated in Figure1. As can be seen, this study is divided into four main sections: (1) Introduction of forces entering the railway track (longitudinal, lateral and vertical), (2) Resistance of forces entering the track, (3) Introduction of laboratory apparatuses of railway tracks in different scales (large and small scales), and (4) TOPSIS analysis according to the weight values related to the details and performance of laboratory methods for better decision making in order to select more efficient apparatuses.Figure 1
The structure of the present study.


## 2. Forces on the Railway Panel

Classification of forces on railways in terms of the direction of force application is the most common method according to which different forces on railways can be classified into three categories:(i)
Vertical forces(ii)
Longitudinal forces (horizontal force along the railways)(iii)
Lateral forces (horizontal and perpendicular to the railway track)Another method is also used in the classification of applied forces on the railway, according to which the applied forces are classified into two categories: static and dynamic. Static forces are mainly due to the weight of railway vehicles. However, it should be noted that due to the transient nature of the forces and the existence of geometric and structural defects in railways and railway vehicles, the nature of the vertical and lateral forces on the railway is dynamic. Figure2 shows the classification of applied forces on the railway. The most important factors influencing the emergence of these dynamic effects include the following [12–16]:(i)
Horizontal and vertical roughness of the track(ii)
Unequal stiffness at different sections of the track due to variable properties and unequal settlements of ballast and subgrade(iii)
Discontinuity of the track in the place of welds, joints and track apparatus(iv)
Wave-shaped deformation on the rail surface(v)
Natural vibrations and rocking motion of railway vehicles(vi)
Disadvantages of the wheel profile, including the deviation of its circumference from the shape of a complete circle, local incision, etc.(vii)
Unequal distribution of the weight of locomotives and wagons between axles or wheels due to height difference in the longitudinal direction of the track(viii)
Braking that increases the load on the front axles of the locomotive and reduces the load on other axlesFigure 2
The classification of applied forces on the railway.


### 2.1. Vertical Forces

Vertical forces are mainly due to the wheel force of passing railway vehicles, the transfer of which is shown in Figure3. Figure 4 also shows the vertical load distribution. The vertical force applied to each railway is obtained by dividing the total weight of the railway vehicles by the number of wheels. Due to the transient nature of the forces and the existence of defects in railways and railway vehicles, the nature of the vertical forces on the railway is dynamic. But considering these dynamic effects and determining the exact amount of forces applied to the railway track requires a lot of time. Hence, for design purposes, vertical forces are applied to the track quasi-statically. Thus, the static force of the wheel of railway vehicles is multiplied by an incremental coefficient called the dynamic impact, and the result is used as a quasi-static force of the wheel in which dynamic effects are considered in the calculations:(1)P=φ×PS,Where P illustrates the dynamic force of the wheel (kN), PS indicates the static force of the wheel of railway vehicles, φ is the dynamic impact factor.Figure 3
Method of transferring vertical forces of the wheel to railway components.Figure 4
Distribution of the vertical force on the railway track.


### 2.2. Longitudinal Forces

In general, the factors that create longitudinal forces include temperature changes due to rail contraction and expansion, acceleration and braking forces in locomotives and wagons, forces applied to the railway track on slopes and ups and downs, forces applied to railway switches and turnouts, forces applied to the track at the beginning and end of bridges and tunnels, the aerodynamic forces against the movement of trains, the longitudinal force due to wheel-rail contact at the curves, the forces created at the transmission points, and the forces due to the rail wave irregularity. Figure5 shows the longitudinal force distribution.Figure 5
Distribution of the longitudinal force on the railway track.


#### 2.2.1. Longitudinal Forces due to Rail Contraction and Expansion

Continuous welded rail (CWR) track is designed to have a constant length in the range of temperature changes. These railways show different behaviors under two critical temperatures, one is the buckling of the railway track during the heat and the other is the failure of the rail during coldness. CWR track tends to move laterally in a section to release a uniform longitudinal force of the rail for the nonuniform balance of force in the railway track. The longitudinal force of the railway track includes the resistance of the rail connection system to the sleeper for preventing the movement of the rail and the ballast resistance in order to prevent the longitudinal movement of the sleeper. If the railway track is not properly bounded in the longitudinal direction, nonuniform conditions will be created in the track that will increase the track’s tendency for buckling. In addition, the longitudinal resistance of the track is very efficient in analyzing the track buckling probability.


#### 2.2.2. Traction and Braking Forces

Traction and braking forces transmit a large amount of force to the train wheels due to the high weight and speed of the train. These forces result in the application of significant longitudinal and vertical forces to the railway track, which create a force distribution. Due to the short distance between the locomotive wheels, after the train passes, the loading effects of each wheel reach the previous and subsequent wheels, and as a result, the traction created in the rail at one point of the railway track is shown in Figure6. As can be seen, due to the passage of the train, a very longitudinal force has been created in a few sleepers ahead of the train.Figure 6
Rail stresses under the force due to train movement.


#### 2.2.3. Longitudinal Forces at Railway Switches and Turnouts

In the collision of the CWR track with railway switches and turnouts, measures should be taken to prevent the transmission of longitudinal forces of the railway track in moving parts because these forces disrupt and damage the mechanism of the moving components of the railway track. This can be done by using an expansion joint at the end of the CWR track or reinforcement by using rails with extended wings that can be used on the switches and turnouts themselves [1]. If the longitudinal force is not controlled in these parts, the railway switches, which are considered as sensitive parts of the railway and have high functional accuracy due to their moving components, will be damaged and due to their incomplete performance, the safety of the railway track, when passing the railway vehicle, will be decreased. Figure 7 shows the shear tab in the railway switch structure for longitudinal control and restraint.Figure 7
Creep tabs designed on the railway switch sliding plates.


#### 2.2.4. Longitudinal Forces in the Transition Areas and Two Ends of the Bridge

In this case, contrary to the above, it is necessary to allow the rail to slide at the beginning and end of the bridge and transition areas, such as tunnel entrances, due to changes in the vertical stiffness in a controlled manner in its longitudinal direction. The topical application of fastening that provides zero longitudinal resistance (ZLR) or significantly reduces resistance can be a key part of structural design, especially on large bridges with no ballast railway and the use of modern transition systems. Due to the possible consequences of rail failure, a series of precautions are necessary [17].


#### 2.2.5. Longitudinal Forces due to Slope

This longitudinal component of the force is obtained from the weight of the railway vehicle as it crosses the slope. If the track is without slope, no longitudinal force due to the effect of gravity applies to the railway track, and if the train is on a slope, the weight component of the railway vehicle along the track is shown in Figure8. As can be seen, the amount of longitudinal force due to the weight of the train on the slope can be calculated from (2) [18]. Due to the low slope of the railway track, the angle is very small. So,(2)Fg=P×sinω=P×tanω=P×i,where i is the value of the longitudinal slope of the railway track.Figure 8
Railway vehicle diagram to calculate the longitudinal force due to slope.


#### 2.2.6. Longitudinal Forces due to Curve Effects in the Track

As the train passes through the parts of the railway track that are in the curve, the track is subjected to longitudinal and lateral forces due to the wheel and rail cap contact. The most important factors affecting the amount of longitudinal force applied to the railway track are as follows [18]:(i)
Friction between the wheel rim and rail cap(ii)
Slipping of the wheel on the rail due to the rigidity of the bogie frameThe amount of longitudinal force due to these factors is calculated from(3)FC=KR,where Fc is the longitudinal force created by the load on the curve, k is a parameter between 500 and 1200 that its average value is 800, and R is the radius of the curve.


#### 2.2.7. Longitudinal Force due to Aerodynamic Resistance to Train Movement

Another longitudinal force created in railways is the resistance due to air impact with a moving rail vehicle. Overcoming this longitudinal resistance requires sufficient traction force from the locomotive, and as a result, friction between the wheel and the rail exerts a longitudinal force on the rail. As the train enters the tunnel quickly, the air in front of train condenses to a certain length of the tunnel, and as it enters, a vacuum is created behind it. The amount of aerodynamic force is proportional to the square of train speed, which can be calculated from (4) [19](4)Fd=C×V2,where C is a constant value determined according to the conditions of the railway track and the geometrical characteristics of the railway fleet from(5)C=K1×S+K2×p×L.In which the first term expresses the aerodynamic resistance created on the front and rear surfaces of the train and the second term refers to the aerodynamic resistance created in the height level of the railway surface below the train body.K1 and K2 are coefficients, S is the surface area of the front of the train, p is the circumference of the fleet body, which is below the railway height level.


### 2.3. Lateral Forces

Figure9 shows the lateral forces applied to the railway, which are perpendicular to the axis of railway track. They are due to the lateral components of the friction force of rail and wheel. The source of wheel’s lateral force is the train reaction to changes in geometry, motion due to bogie instability at a higher speed and the centrifugal force in the curves, which are as follows [20]:(i)
The lateral component of the friction force of rail and wheel(ii)
The lateral force exerted to the rail from the wheel rim, especially in the curves(iii)
Rail buckling due to changes in the axial force of the rail, including changes in rail temperatureFigure 9
Components of lateral forces applied on the railway.As illustrated in Figure10, the railway track displacement force includes the lateral axial forces of the vehicle (L) and the thermal compressive loads of the rail (P0). The lateral axial force is due to the force created in the curve and the dynamic increase of the initial deviation of the railway track in the lateral direction. In contrast to these forces, the track reaction force (S) appears as the lateral resistance to thermal and vehicle loads. In general, the ballast resistance is due to the friction under and beside the sleeper and the limiting force of the shoulder ballast along with the crib ballast, which is the same as the reaction force S. If the combination of L and P0 exceeds the value of S, a permanent lateral displacement occurs in the railway track. In other words, lateral displacement in the railway track is a permanent deviation of a section of the railway track due to lateral forces resulting from the passage of railway vehicles and can lead to unsafe conditions.Figure 10
Lateral forces of the railway track and its reaction.As can be seen in Figure11, the rail due to the flexural stiffness in the horizontal plane and fastening due to the torsional stiffness in the horizontal plane has a significant role in providing the lateral resistance of railway track. During the buckling phenomenon, the portion of rail resistance is reduced to zero because the rail itself creates the force that causes lateral displacement. As a result, the main source of supply for the lateral resistance of the railway track is the interactions between the sleeper and the ballast.Figure 11
Contribution of lateral stiffness of pavement components.


## 2.1. Vertical Forces

Vertical forces are mainly due to the wheel force of passing railway vehicles, the transfer of which is shown in Figure3. Figure 4 also shows the vertical load distribution. The vertical force applied to each railway is obtained by dividing the total weight of the railway vehicles by the number of wheels. Due to the transient nature of the forces and the existence of defects in railways and railway vehicles, the nature of the vertical forces on the railway is dynamic. But considering these dynamic effects and determining the exact amount of forces applied to the railway track requires a lot of time. Hence, for design purposes, vertical forces are applied to the track quasi-statically. Thus, the static force of the wheel of railway vehicles is multiplied by an incremental coefficient called the dynamic impact, and the result is used as a quasi-static force of the wheel in which dynamic effects are considered in the calculations:(1)P=φ×PS,Where P illustrates the dynamic force of the wheel (kN), PS indicates the static force of the wheel of railway vehicles, φ is the dynamic impact factor.Figure 3
Method of transferring vertical forces of the wheel to railway components.Figure 4
Distribution of the vertical force on the railway track.


## 2.2. Longitudinal Forces

In general, the factors that create longitudinal forces include temperature changes due to rail contraction and expansion, acceleration and braking forces in locomotives and wagons, forces applied to the railway track on slopes and ups and downs, forces applied to railway switches and turnouts, forces applied to the track at the beginning and end of bridges and tunnels, the aerodynamic forces against the movement of trains, the longitudinal force due to wheel-rail contact at the curves, the forces created at the transmission points, and the forces due to the rail wave irregularity. Figure5 shows the longitudinal force distribution.Figure 5
Distribution of the longitudinal force on the railway track.


### 2.2.1. Longitudinal Forces due to Rail Contraction and Expansion

Continuous welded rail (CWR) track is designed to have a constant length in the range of temperature changes. These railways show different behaviors under two critical temperatures, one is the buckling of the railway track during the heat and the other is the failure of the rail during coldness. CWR track tends to move laterally in a section to release a uniform longitudinal force of the rail for the nonuniform balance of force in the railway track. The longitudinal force of the railway track includes the resistance of the rail connection system to the sleeper for preventing the movement of the rail and the ballast resistance in order to prevent the longitudinal movement of the sleeper. If the railway track is not properly bounded in the longitudinal direction, nonuniform conditions will be created in the track that will increase the track’s tendency for buckling. In addition, the longitudinal resistance of the track is very efficient in analyzing the track buckling probability.


### 2.2.2. Traction and Braking Forces

Traction and braking forces transmit a large amount of force to the train wheels due to the high weight and speed of the train. These forces result in the application of significant longitudinal and vertical forces to the railway track, which create a force distribution. Due to the short distance between the locomotive wheels, after the train passes, the loading effects of each wheel reach the previous and subsequent wheels, and as a result, the traction created in the rail at one point of the railway track is shown in Figure6. As can be seen, due to the passage of the train, a very longitudinal force has been created in a few sleepers ahead of the train.Figure 6
Rail stresses under the force due to train movement.


### 2.2.3. Longitudinal Forces at Railway Switches and Turnouts

In the collision of the CWR track with railway switches and turnouts, measures should be taken to prevent the transmission of longitudinal forces of the railway track in moving parts because these forces disrupt and damage the mechanism of the moving components of the railway track. This can be done by using an expansion joint at the end of the CWR track or reinforcement by using rails with extended wings that can be used on the switches and turnouts themselves [1]. If the longitudinal force is not controlled in these parts, the railway switches, which are considered as sensitive parts of the railway and have high functional accuracy due to their moving components, will be damaged and due to their incomplete performance, the safety of the railway track, when passing the railway vehicle, will be decreased. Figure 7 shows the shear tab in the railway switch structure for longitudinal control and restraint.Figure 7
Creep tabs designed on the railway switch sliding plates.


### 2.2.4. Longitudinal Forces in the Transition Areas and Two Ends of the Bridge

In this case, contrary to the above, it is necessary to allow the rail to slide at the beginning and end of the bridge and transition areas, such as tunnel entrances, due to changes in the vertical stiffness in a controlled manner in its longitudinal direction. The topical application of fastening that provides zero longitudinal resistance (ZLR) or significantly reduces resistance can be a key part of structural design, especially on large bridges with no ballast railway and the use of modern transition systems. Due to the possible consequences of rail failure, a series of precautions are necessary [17].


### 2.2.5. Longitudinal Forces due to Slope

This longitudinal component of the force is obtained from the weight of the railway vehicle as it crosses the slope. If the track is without slope, no longitudinal force due to the effect of gravity applies to the railway track, and if the train is on a slope, the weight component of the railway vehicle along the track is shown in Figure8. As can be seen, the amount of longitudinal force due to the weight of the train on the slope can be calculated from (2) [18]. Due to the low slope of the railway track, the angle is very small. So,(2)Fg=P×sinω=P×tanω=P×i,where i is the value of the longitudinal slope of the railway track.Figure 8
Railway vehicle diagram to calculate the longitudinal force due to slope.


### 2.2.6. Longitudinal Forces due to Curve Effects in the Track

As the train passes through the parts of the railway track that are in the curve, the track is subjected to longitudinal and lateral forces due to the wheel and rail cap contact. The most important factors affecting the amount of longitudinal force applied to the railway track are as follows [18]:(i)
Friction between the wheel rim and rail cap(ii)
Slipping of the wheel on the rail due to the rigidity of the bogie frameThe amount of longitudinal force due to these factors is calculated from(3)FC=KR,where Fc is the longitudinal force created by the load on the curve, k is a parameter between 500 and 1200 that its average value is 800, and R is the radius of the curve.


### 2.2.7. Longitudinal Force due to Aerodynamic Resistance to Train Movement

Another longitudinal force created in railways is the resistance due to air impact with a moving rail vehicle. Overcoming this longitudinal resistance requires sufficient traction force from the locomotive, and as a result, friction between the wheel and the rail exerts a longitudinal force on the rail. As the train enters the tunnel quickly, the air in front of train condenses to a certain length of the tunnel, and as it enters, a vacuum is created behind it. The amount of aerodynamic force is proportional to the square of train speed, which can be calculated from (4) [19](4)Fd=C×V2,where C is a constant value determined according to the conditions of the railway track and the geometrical characteristics of the railway fleet from(5)C=K1×S+K2×p×L.In which the first term expresses the aerodynamic resistance created on the front and rear surfaces of the train and the second term refers to the aerodynamic resistance created in the height level of the railway surface below the train body.K1 and K2 are coefficients, S is the surface area of the front of the train, p is the circumference of the fleet body, which is below the railway height level.


## 2.2.1. Longitudinal Forces due to Rail Contraction and Expansion

Continuous welded rail (CWR) track is designed to have a constant length in the range of temperature changes. These railways show different behaviors under two critical temperatures, one is the buckling of the railway track during the heat and the other is the failure of the rail during coldness. CWR track tends to move laterally in a section to release a uniform longitudinal force of the rail for the nonuniform balance of force in the railway track. The longitudinal force of the railway track includes the resistance of the rail connection system to the sleeper for preventing the movement of the rail and the ballast resistance in order to prevent the longitudinal movement of the sleeper. If the railway track is not properly bounded in the longitudinal direction, nonuniform conditions will be created in the track that will increase the track’s tendency for buckling. In addition, the longitudinal resistance of the track is very efficient in analyzing the track buckling probability.


## 2.2.2. Traction and Braking Forces

Traction and braking forces transmit a large amount of force to the train wheels due to the high weight and speed of the train. These forces result in the application of significant longitudinal and vertical forces to the railway track, which create a force distribution. Due to the short distance between the locomotive wheels, after the train passes, the loading effects of each wheel reach the previous and subsequent wheels, and as a result, the traction created in the rail at one point of the railway track is shown in Figure6. As can be seen, due to the passage of the train, a very longitudinal force has been created in a few sleepers ahead of the train.Figure 6
Rail stresses under the force due to train movement.


## 2.2.3. Longitudinal Forces at Railway Switches and Turnouts

In the collision of the CWR track with railway switches and turnouts, measures should be taken to prevent the transmission of longitudinal forces of the railway track in moving parts because these forces disrupt and damage the mechanism of the moving components of the railway track. This can be done by using an expansion joint at the end of the CWR track or reinforcement by using rails with extended wings that can be used on the switches and turnouts themselves [1]. If the longitudinal force is not controlled in these parts, the railway switches, which are considered as sensitive parts of the railway and have high functional accuracy due to their moving components, will be damaged and due to their incomplete performance, the safety of the railway track, when passing the railway vehicle, will be decreased. Figure 7 shows the shear tab in the railway switch structure for longitudinal control and restraint.Figure 7
Creep tabs designed on the railway switch sliding plates.


## 2.2.4. Longitudinal Forces in the Transition Areas and Two Ends of the Bridge

In this case, contrary to the above, it is necessary to allow the rail to slide at the beginning and end of the bridge and transition areas, such as tunnel entrances, due to changes in the vertical stiffness in a controlled manner in its longitudinal direction. The topical application of fastening that provides zero longitudinal resistance (ZLR) or significantly reduces resistance can be a key part of structural design, especially on large bridges with no ballast railway and the use of modern transition systems. Due to the possible consequences of rail failure, a series of precautions are necessary [17].


## 2.2.5. Longitudinal Forces due to Slope

This longitudinal component of the force is obtained from the weight of the railway vehicle as it crosses the slope. If the track is without slope, no longitudinal force due to the effect of gravity applies to the railway track, and if the train is on a slope, the weight component of the railway vehicle along the track is shown in Figure8. As can be seen, the amount of longitudinal force due to the weight of the train on the slope can be calculated from (2) [18]. Due to the low slope of the railway track, the angle is very small. So,(2)Fg=P×sinω=P×tanω=P×i,where i is the value of the longitudinal slope of the railway track.Figure 8
Railway vehicle diagram to calculate the longitudinal force due to slope.


## 2.2.6. Longitudinal Forces due to Curve Effects in the Track

As the train passes through the parts of the railway track that are in the curve, the track is subjected to longitudinal and lateral forces due to the wheel and rail cap contact. The most important factors affecting the amount of longitudinal force applied to the railway track are as follows [18]:(i)
Friction between the wheel rim and rail cap(ii)
Slipping of the wheel on the rail due to the rigidity of the bogie frameThe amount of longitudinal force due to these factors is calculated from(3)FC=KR,where Fc is the longitudinal force created by the load on the curve, k is a parameter between 500 and 1200 that its average value is 800, and R is the radius of the curve.


## 2.2.7. Longitudinal Force due to Aerodynamic Resistance to Train Movement

Another longitudinal force created in railways is the resistance due to air impact with a moving rail vehicle. Overcoming this longitudinal resistance requires sufficient traction force from the locomotive, and as a result, friction between the wheel and the rail exerts a longitudinal force on the rail. As the train enters the tunnel quickly, the air in front of train condenses to a certain length of the tunnel, and as it enters, a vacuum is created behind it. The amount of aerodynamic force is proportional to the square of train speed, which can be calculated from (4) [19](4)Fd=C×V2,where C is a constant value determined according to the conditions of the railway track and the geometrical characteristics of the railway fleet from(5)C=K1×S+K2×p×L.In which the first term expresses the aerodynamic resistance created on the front and rear surfaces of the train and the second term refers to the aerodynamic resistance created in the height level of the railway surface below the train body.K1 and K2 are coefficients, S is the surface area of the front of the train, p is the circumference of the fleet body, which is below the railway height level.


## 2.3. Lateral Forces

Figure9 shows the lateral forces applied to the railway, which are perpendicular to the axis of railway track. They are due to the lateral components of the friction force of rail and wheel. The source of wheel’s lateral force is the train reaction to changes in geometry, motion due to bogie instability at a higher speed and the centrifugal force in the curves, which are as follows [20]:(i)
The lateral component of the friction force of rail and wheel(ii)
The lateral force exerted to the rail from the wheel rim, especially in the curves(iii)
Rail buckling due to changes in the axial force of the rail, including changes in rail temperatureFigure 9
Components of lateral forces applied on the railway.As illustrated in Figure10, the railway track displacement force includes the lateral axial forces of the vehicle (L) and the thermal compressive loads of the rail (P0). The lateral axial force is due to the force created in the curve and the dynamic increase of the initial deviation of the railway track in the lateral direction. In contrast to these forces, the track reaction force (S) appears as the lateral resistance to thermal and vehicle loads. In general, the ballast resistance is due to the friction under and beside the sleeper and the limiting force of the shoulder ballast along with the crib ballast, which is the same as the reaction force S. If the combination of L and P0 exceeds the value of S, a permanent lateral displacement occurs in the railway track. In other words, lateral displacement in the railway track is a permanent deviation of a section of the railway track due to lateral forces resulting from the passage of railway vehicles and can lead to unsafe conditions.Figure 10
Lateral forces of the railway track and its reaction.As can be seen in Figure11, the rail due to the flexural stiffness in the horizontal plane and fastening due to the torsional stiffness in the horizontal plane has a significant role in providing the lateral resistance of railway track. During the buckling phenomenon, the portion of rail resistance is reduced to zero because the rail itself creates the force that causes lateral displacement. As a result, the main source of supply for the lateral resistance of the railway track is the interactions between the sleeper and the ballast.Figure 11
Contribution of lateral stiffness of pavement components.


## 3. Resistance of the Applied Forces

The structure of track should withstand repeated lateral, longitudinal, vertical and torsional forces due to temperature changes and traffic. The combination of these forces creates more critical conditions for a ballast track. In addition, changes in rail temperature create thermal stresses in the rail that makes the steel of the rail expand or contract. Any change in the length of the rail creates internal stresses that usually act as an active force in the direction of the rail length. Without sufficient resistance, the track may buckle in the lateral or vertical direction because of longitudinal compressive forces in the rail, or it may fail due to tensile forces under reduced temperature. The resistance components of a railway track in different directions are shown in Figure12.Figure 12
Resistance parameters of the track include longitudinal, lateral and torsional resistances.


### 3.1. Longitudinal Resistance

The longitudinal resistance of the railway track is the resistance due to the fastenings to the rail and the resistance due to the ballast to the sleeper against the longitudinal movement of the track and the other part is provided by the interactions of the sleeper and the ballast. When the rail is stressed in its longitudinal direction, it causes the rail to shift relative to the sleeper. This displacement depends on the friction of the rail heel, rail seat and pad, and of course, the type of rail connection system to the sleeper in terms of elasticity or rigidity also affects the amount of this displacement. Another part of the track longitudinal resistance is affected by the interactions of sleeper and ballast, which in that case, the ballast granulation, density and tamping play a role in providing this parameter for the track. Crib ballast acts as a passive mass and resists the track displacement during longitudinal loading. As a result, ballast conditions have a significant effect on longitudinal resistance of the railway track. Generally, the factors affecting the longitudinal resistance of the track are fastening type, fastening condition, sleeper type, distance between the sleeper, type and material of ballast granulation, ballast density amount, crib ballast surface, ambient and rail temperatures, ballast moisture and pollution, and railway maintenance conditions [21].


### 3.2. Lateral Resistance

Lateral resistance of railway tracks is another factor affecting the performance and safety of the ballast track. Today, increasing the speed and axial load is possible by maintaining the joints and connection points and thus the use of continuous welding rail (CWR) tracks. Joints in railway tracks are one of the weakest points and due to the destructive effects they have on the pavement and infrastructure, it causes premature deterioration of the railway [10]. Among the advantages of CWR tracks are reducing the maintenance volume and costs, increasing the life of the rail and railway vehicle under the fatigue phenomenon and finally overcoming the weakness of the railway track at the joint. However, sometimes defects such as railway track buckling and railway failure because of thermal stresses of the rail are observed [22, 23].The railway track lateral resistance is the resistance to the lateral forces applied in perpendicular to the main axis of the rail or parallel to the sleepers’ axis that are mainly due to contact forces of wheel and railway, buckling forces, and centrifugal forces in curves [20]. The lateral resistance of railway track is in influence by various factors, including(i)
Weight, type, dimensions and distance between sleepers(ii)
Subgrade grading curve and the amount of ballast density(iii)
Ballast quality between sleepers and ballast shoulder(iv)
The lateral force applied to the railway track due to train movement(v)
Width, height, shoulder slope and ballast thicknessThe railway track lateral resistance is provided by three areas of friction of the sleeper: bottom with ballast, friction around the sleeper with ballast (crib area) and end ballast of sleeper (shoulder) [24, 25]. The friction under the sleeper is affected by the type and weight of sleeper, and the vertical force applied to it, while the high resistance of the shoulder depends more on the geometry of the shoulder because the ballast shoulder prevents the movement of the sleeper due to ballast shear force. The frictional resistance around the sleeper also depends on the length of the sleeper and the height of the sleeper embedded in the ballast.


### 3.3. Creep

Knowing the tolerance and longitudinal resistance provided by the rail-to-sleeper connection system is very important in order to study the creep phenomenon (longitudinal deformation of the railway track due to temperature changes), the effects of longitudinal forces due to braking and acceleration of the train, the effects of longitudinal forces due to the railway fleet crossing in the slopes of the track and so on.Railway engineers are trying to gain a correct understanding of the longitudinal force that causes the rail to move or slip. Also, if a slip occurs, it must be determined whether the slip is between the sleeper and the rail or between the sleeper and the ballast because it is important to understand the potential of the failure mode and which repair mode is easier to repair. The railway track buckling occurs at high temperatures where sleepers can move transversely in the ballast and the rail cannot necessarily move in its seat. This reaction in the underlying soil is similar to the railway track. However, buckling of the railway track occurs when the railway track longitudinal resistance is not equal and the stress occurs only in some sections of CWR tracks. Of course, it should be noted that this parameter also depends on the performance of the fastening system. At low temperatures, fastenings play an important role in preventing rail failure. According to Australian railway reports, the mechanisms that cause creep are movement expansion and contraction as well as traffic [26].Thus, creep can not only change the normal temperature of the railway track, but can also disrupt the track structure and reduce the resistance to lateral forces. The creep resistance measured in the laboratory depends on many factors in the design details of the rail joints to the sleeper, some of which are as follows [17]:(i)
Toe load/clamping force(ii)
The friction coefficient between pad and rail(iii)
The friction coefficient between pad and sleeper(iv)
Shear deformation of sleeper pad(v)
Mechanical interaction between sleeper pad, fastenings and shoulders


## 3.1. Longitudinal Resistance

The longitudinal resistance of the railway track is the resistance due to the fastenings to the rail and the resistance due to the ballast to the sleeper against the longitudinal movement of the track and the other part is provided by the interactions of the sleeper and the ballast. When the rail is stressed in its longitudinal direction, it causes the rail to shift relative to the sleeper. This displacement depends on the friction of the rail heel, rail seat and pad, and of course, the type of rail connection system to the sleeper in terms of elasticity or rigidity also affects the amount of this displacement. Another part of the track longitudinal resistance is affected by the interactions of sleeper and ballast, which in that case, the ballast granulation, density and tamping play a role in providing this parameter for the track. Crib ballast acts as a passive mass and resists the track displacement during longitudinal loading. As a result, ballast conditions have a significant effect on longitudinal resistance of the railway track. Generally, the factors affecting the longitudinal resistance of the track are fastening type, fastening condition, sleeper type, distance between the sleeper, type and material of ballast granulation, ballast density amount, crib ballast surface, ambient and rail temperatures, ballast moisture and pollution, and railway maintenance conditions [21].


## 3.2. Lateral Resistance

Lateral resistance of railway tracks is another factor affecting the performance and safety of the ballast track. Today, increasing the speed and axial load is possible by maintaining the joints and connection points and thus the use of continuous welding rail (CWR) tracks. Joints in railway tracks are one of the weakest points and due to the destructive effects they have on the pavement and infrastructure, it causes premature deterioration of the railway [10]. Among the advantages of CWR tracks are reducing the maintenance volume and costs, increasing the life of the rail and railway vehicle under the fatigue phenomenon and finally overcoming the weakness of the railway track at the joint. However, sometimes defects such as railway track buckling and railway failure because of thermal stresses of the rail are observed [22, 23].The railway track lateral resistance is the resistance to the lateral forces applied in perpendicular to the main axis of the rail or parallel to the sleepers’ axis that are mainly due to contact forces of wheel and railway, buckling forces, and centrifugal forces in curves [20]. The lateral resistance of railway track is in influence by various factors, including(i)
Weight, type, dimensions and distance between sleepers(ii)
Subgrade grading curve and the amount of ballast density(iii)
Ballast quality between sleepers and ballast shoulder(iv)
The lateral force applied to the railway track due to train movement(v)
Width, height, shoulder slope and ballast thicknessThe railway track lateral resistance is provided by three areas of friction of the sleeper: bottom with ballast, friction around the sleeper with ballast (crib area) and end ballast of sleeper (shoulder) [24, 25]. The friction under the sleeper is affected by the type and weight of sleeper, and the vertical force applied to it, while the high resistance of the shoulder depends more on the geometry of the shoulder because the ballast shoulder prevents the movement of the sleeper due to ballast shear force. The frictional resistance around the sleeper also depends on the length of the sleeper and the height of the sleeper embedded in the ballast.


## 3.3. Creep

Knowing the tolerance and longitudinal resistance provided by the rail-to-sleeper connection system is very important in order to study the creep phenomenon (longitudinal deformation of the railway track due to temperature changes), the effects of longitudinal forces due to braking and acceleration of the train, the effects of longitudinal forces due to the railway fleet crossing in the slopes of the track and so on.Railway engineers are trying to gain a correct understanding of the longitudinal force that causes the rail to move or slip. Also, if a slip occurs, it must be determined whether the slip is between the sleeper and the rail or between the sleeper and the ballast because it is important to understand the potential of the failure mode and which repair mode is easier to repair. The railway track buckling occurs at high temperatures where sleepers can move transversely in the ballast and the rail cannot necessarily move in its seat. This reaction in the underlying soil is similar to the railway track. However, buckling of the railway track occurs when the railway track longitudinal resistance is not equal and the stress occurs only in some sections of CWR tracks. Of course, it should be noted that this parameter also depends on the performance of the fastening system. At low temperatures, fastenings play an important role in preventing rail failure. According to Australian railway reports, the mechanisms that cause creep are movement expansion and contraction as well as traffic [26].Thus, creep can not only change the normal temperature of the railway track, but can also disrupt the track structure and reduce the resistance to lateral forces. The creep resistance measured in the laboratory depends on many factors in the design details of the rail joints to the sleeper, some of which are as follows [17]:(i)
Toe load/clamping force(ii)
The friction coefficient between pad and rail(iii)
The friction coefficient between pad and sleeper(iv)
Shear deformation of sleeper pad(v)
Mechanical interaction between sleeper pad, fastenings and shoulders


## 4. Scaled Laboratory Apparatuses

Various studies have been performed on railway tracks in laboratory conditions at different scales. What details of the railway tracks are to be loaded is the basis for the classification of laboratory apparatuses. This classification includes three categories of loading tests on the ballast, track panel and roller rig, and their range includes apparatuses with scales of 1 : 1 to 1 : 20.The first category includes loading tests on the ballast and subballast tracks, the main tests performed of which are according to Table1. These tests determine various parameters such as permanent deformation of the track, axial strains, resistance and deformation of the shoulder, water level effect, cumulative and elastic deformations, modulus of resilience and other influential parameters related to the ballast. The advantage of these tests is that they can focus on the behavioral characteristics of the ballast and the materials involved, but the ballast behavior is less evaluated under the influence of rails and sleepers. Also, the effects of the interaction between the rail and the wheel on the ballast are not considered.Table 1
Ballast and subgrade test methods.
Test set up nameScaleLocationPurposesSmall-scale triaxial1 : 5Brazil(i) Permanent deformation(ii) Degradation behavior during monotonic and cyclic vertical loadings(iii) Permanent axial strainEnglandCanadaUnited StatesUnited StatesEnglandAustraliaFranceLarge-scale triaxial1 : 1United KingdomNorwayIranSleeper and ballasted tracks1 : 3United Kingdom(i) Resistance of sleepers(ii) Shoulder deformationMoving-wheel loading1 : 11 : 5ChinaJapan(i) The effect of water level(ii) Effect of wetting-drying cycles(iii) Dynamic stresses of train loadings(iv) Cumulative settlement(v) Elastic deformation(vi) Cumulative strainBallast box1 : 1AustraliaChinaIndiaIran(i) Lateral displacement(ii) Lateral and vertical strain(iii) Resilient modulus(iv) Ballast permanent deformationDirect shear test machine1 : 1Iran(i) Shear stress(ii) Lateral and longitudinal displacementStone blowing1 : 1United Kingdom(i) Behavior of ballast layer(ii) Settlement amountPlate load tests (PLT)1 : 1Iran(i) Stiffness(ii) Permanent deformation(iii) SettlementThe second category of tests relates to tests that subject the entire railway track in all detail to the static and dynamic loads, as shown in Table2. Many of these tests are at 1 : 1 scale and the same size as the real railway tracks with only a limited number of sleepers. But some of these tests have been constructed on a much smaller scale, which in turn reduces the construction cost of laboratory apparatuses. Studies have shown that these tests, both on a large and small scale, simulate only a fraction of the forces applied on the railway track, and this can be the basis for the construction of similar apparatuses on a smaller scale (with lower construction costs) that simulate all three vertical, longitudinal, and lateral loads.Table 2
Railway track panel test methods.
Test set up nameScaleLocationPurposesFull scaleFatigue test1 : 1Spain(i) Fatigue life(ii) Displacement and stiffness(iii) Probability of failureRailway test facility1 : 1United Kingdom(i) Permanent axial strain(ii) Resilient modulus(iii) Volumetric strainTrack longitudinal resistance1 : 1Iran(i) Track longitudinal resistance(ii) Track stiffness(iii) Longitudinal displacementLateral track panel test (LTPT)1 : 1Iran(i) Track lateral resistance(ii) Lateral displacementSingle tie push test (STPT)1 : 1Poland(i) Sleeper lateral resistanceIranPendulum loading test apparatus1 : 1Iran(i) Sleeper-ballast interaction force(ii) Sleeper lateral displacementSmall scaleSingle sleeper track1 : 3United Kingdom(i) Displacement(ii) Ballast shoulder resistanceBallasted railway track with three sleepers1 : 3France(i) Settlement of ballasted tracks(ii) Displacements affected by accelerations and pressures(iii) Dynamic behavior(iv) Depth settlement(v) Effect of wetting-drying cycles cumulative deformation(vi) Permanent axial deformationScaled railway track models1 : 5Japan(i) Horizontal displacement(ii) Vertical displacement(iii) Lateral resistance of ballast track(iv) Effect of sleepers spacing(v) Effect of types of sleepers(vi) Horizontal loading1 : 5United Kingdom(i) Sound radiation affected by vertical and lateral motion(ii) Sound pressure level(iii) Average mobility1 : 8.5Japan(i) Rolling noise and rail vibration(ii) Frequency spectra(iii) Weighted sound pressure levelAccording to Table3, roller rigs are the third category of tests with railway track scales, which deal with the interaction between rails and wheels. Parameters such as lateral displacement, mechanical characteristics of different types of bogies and the effect of their behavior on the track, rail fatigue under wheel crossings, effects of curved railways, effect of speed on the behavior between the wheel and the rail and other parameters are among the results obtained from these studies on different scales.Table 3
Roller rig test methods.
Test set up nameScaleLocationPurposesRoller rigs for studying railway vehicle dynamics(i) Lateral displacement detection(ii) Fatigue mechanism analysis(iii) Ballast affected by various parameters(iv) Wheel lateral forces(v) Lateral displacement(vi) Vibration(vii) Effect of track curvature(viii) Vertical acceleration and force(ix) Roll angular speedFull-scale roller rig1 : 1ItalyScaled roller test rig1 : 5ItalyRoller rig1 : 1GermanyVehicle/Bogie roller rig1 : 1GermanyFull-scale roller rig1 : 1ChinaScale roller rig1 : 5UKReduced-scale rolling rig1 : 3NetherlandNaples vehicle roller rig1 : 1ItalyINRETS bogie test rig1 : 4FranceFull-scale roller rig1 : 1TurkeyRoller rig1 : 5GermanyActive steering vehicle and curved track1 : 5KoreaScale curve track1 : 5KoreaActive steering vehicle1 : 5KoreaScaled track1 : 10SpainScaled bogie1 : 5GermanyFull-scale roller rig1 : 1ItalySmall-scale test rig1 : 5NetherlandsDTR dynamic train bogie test rig1 : 1SpainRoller rigs for wheel-rail contact investigationISVR wheel-rail test rig1 : 5UKWheel-rail test rig1 : 1UKWheel set roller rig1 : 1GermanyWheel test rig1 : 1GermanyWheel/rail tribological rolling rig1 : 4ChinaUNSW wheel-rail rolling rig1 : 5AustraliaScaled wheel on railway track-wheel test rig1 : 3UKFull-scale rail wheel test rig1 : 1AustriaScaled wheelset roller stand1 : 5JapanFull-scale roller stand1 : 1JapanSmall-scaled derailment simulator1 : 5KoreaScaled-bogie test bench1 : 5BelgiumRoller-rig1 : 5ItalyBU300 roller rig1 : 1ItalyScaled bogie on rollers1 : 5NetherlandTwo-disc test rig1 : 5AustraliaTwin-disc rolling-sliding machine1 : 2Japan


### 4.1. Ballast and Subgrade Tests

The infrastructure and ballast layer have a significant role in loading and resistance of a railway track, and therefore, researchers follow the methods to enhance the performance and loading of railway tracks through the reduction of lateral movement, permanent deformation, and settlement of railway system. These include adding crumbs and sand to the ballast and placing the geogrid in the ballast mixture. To evaluate and determine the loading amount of the ballast layer, it should be fulfilled various tests that, according to purposes and laboratory investment cost, different ballast apparatuses with the various scales have been built, which are described as follows.


#### 4.1.1. Large and Small Scale Triaxial Tests

This apparatus can be used to determine the permanent deformation, degradation behavior during monotonic and cyclic vertical loadings, and permanent axial strain, which includes full scale and small scale. Figure13(a) illustrates the full-scale test setup. A railway track section is constructed with ballast, subballast, and subgrade containing crushed rocks and confined with a rigid experiment box with 1.5 m wide and 3 m long, which all are rested on a concrete floor. Under the upper layer of ballast, the support layers are neither changed nor removed within the complete experiment. This test apparatus has been built in Iran, Norway, and the United Kingdom on the full scale. Also, the small scale of this test is shown in Figure 13(b), which usually are built on a 1 : 5 scale. Different countries such as United States, Canada, Brazil, United Kingdom, Australia, and France use this method to evaluate the ballast layer. This apparatus uses a hydraulic mechanism to simulate loading on the ballast layer in the laboratory scale and its load frame is able to apply both cyclic and monotonic loads to a specimen. The maximum force that is applicable is 100 kN that is equivalent to a 5659 kPa deviatoric stress on the specimen (15 cm diameter). The axial loading is exerted on the specimen by the bottom movement against the stationary loading cell at the top of the specimen. The confinement method used was a vacuum system that is sufficient for materials placed relatively close to the surface, and therefore under relatively low confinement. The maximum confinement available using vacuum is 95 kPa (close to 1 atm).Figure 13
Small-scale cylindrical triaxial apparatus: (a) full-scaled [27], (b) small-scaled [28].
(a)(b)


#### 4.1.2. Sleeper and Ballast Track

The experiment model a 1/3 size sleeper end being pushed gently to a shoulder constructed with a scaled ballast. The displacement is controlled using linear variable displacement transducer (LVDT) and optically, and the resistance on the sleeper end is examined using a loading cell. The sample ballast shoulder is confined between vertical wooden borders located appropriately beyond the expected amount of failure mechanisms, as illustrated in the overall view of the experiment setup displayed in Figure14. Therefore, the testing apparatus boundaries have not any effect on outcomes. The ballast bed extends to 11 cm depth under the sample sleeper end bottom, related to 33 cm at full-scale. A rough sandpaper mat at the base of ballasts prevents ballast materials from sliding along the junction with the wooden surface on which experiments are performed.Figure 14
The sleeper and ballast track apparatus [29].


#### 4.1.3. Moving-Wheel Loading

This test has been used to investigate the performance of ballast layer and subgrade on two scales: full and small (1 : 5), which the full one is shown according to Figure15. Also, it can be conducted to examine the elastic deformation, dynamic soil pressure, vibration speed and cumulative settlements bellow simulated dynamic train loading. In full-scale condition, the subgrade constituted by unbound granular substances has a 2.7 × 5 × 13.1 m dimensions in depth, width and length, respectively. The dynamic train loadings are simulated by the set of phase difference of contiguous actuators (8 actuators in tests) and the speed of simulated maximum train is 360 km/h [30, 31]. But in small-scale condition, all dimensions were changed into 1 : 5 scale.Figure 15
The moving-wheel loading test [32].


#### 4.1.4. Stone Blowing

The apparatus presented in Figure16 is used to determine the settlement amount and the ballast layer behavior affected by repeating loading. A rubber mat with the 1 cm thickness and 5000 kPa reported elastic modulus is glued to this box bottom for simulating the actions of track subgrade, on which the experiment substance is placed. Substances with a field size are utilized with the main ballast bed composed with 5 cm single size angular medium gravels and 3 smaller single size gravels that are utilized for the upper stone layer. The test bed is loaded using a sample reinforced concrete sleeper with dimensions 60 cm by 10 cm by 10 cm in length, width and deep, respectively, which is constrained by 2 arms bolted upon the sleeper and connected to the apparatus sides. These arms are articulated to preclude movements other than vertical direction and simulate the constraint of rail and sleeper fastenings in actual railway tracks. The sleeper is under the load of a hydraulic ram using a steel ball and seat arrangement on the sleeper top. This ram reacts against the top of a pair of steel beams that connectivity with it permits the ram to be angled in any direction from the vertical, however, in experiments presented here, only vertical loadings are applied. The hydraulic ram is fed using a high-capacity pump and is a servo-hydraulic computer-controlled valve for operating in various modes. A sine wave loading-unloading cycle is used in the experiments. Integrated with the hydraulic ram are transducers to calculate the loading on the sleeper and sleeper displacements corresponding to the start situation. The sample sleeper rigidity is such that no bending happens under loadings.Figure 16
Stone blowing [33].


#### 4.1.5. Direct Shear Test Machine

Inasmuch as ballast substances are constituted with coarse materials and aggregates, a large-scale direct shear test machine can be applied to evaluate the shear stress and horizontal displacement of railway ballast materials. A sample of this machine was fabricated and designed at Iran University of Science and Technology, in which there are two shear boxes, the upper and the lower. The lower box has 440× 540 mm dimensions in width and length, respectively. The upper shear box has a square form and 440 mm length. Both boxes have the same height of 180 mm. Figure 17(a) illustrates the side view of the machine. Also, Figure 17(b) indicates the top and bottom shear boxes. Ballast specimens are placed in 2 layers in these shear boxes. The ballast is compacted in the layers similar to the field by the use of a hand tamper to achieve the 17.5 kN/m3 dry density [34]. Based on ASTM D3080, direct shear tests are performed. For all tests, the upper shear box movements are constrained in longitudinal and lateral. The lower shear box is moved in the longitudinal direction at a constant rate [34]. Vertical stress is exerted on the ballast surface in the upper shear box using a Jack which transfers the normal forces to a steel rigid plate placed on the ballast top. The vertical stress remains constant in the tests. 50, 100, 150, and 200 kPa normal pressures are exerted to the specimens within the tests. By the use of LVDT and loading cells, the horizontal and vertical displacements and the exerted forces are concurrently calculated. All sensors are connected to a data logger that records information in the tests.Figure 17
Direct shear test machine [35]: (a) overall view of the machine, (b) dimension of the boxes.
(a)(b)


#### 4.1.6. Plate Load Test (PLT)

This apparatus is used to evaluate stiffness, permanent deformation, settlement of the ballast layer of the railway. In order to conduct PLT, ballast substances are placed in a large steel chamber, which has dimensions of 1200, 1200, and 1000 mm in length, width, and depth, respectively, as illustrated in Figure18. For PLT, 3 clean ballast layers should be placed in the chamber, the equal thickness of which is 300 mm that is close to the depth of the ballast layer in the railway field. The 17.5 kN/m3 density is achieved for the dry density of clean ballasts. A hydraulic Jack with 30 ton capacity is usually utilized in order to apply loads to the plate. Moreover, a rigid frame fixed to the laboratory floor strongly is utilized for supporting the Jack for vertical loads. The chamber is mounted bellow the rigid frame beam. The data recording setup and steel chamber are other related apparatuses. A rigid steel plate in a square form is utilized as the loading plate that is placed on the ballast surface, the dimensions of which are 15 × 30 cm in thickness and width, respectively.Figure 18
Plate load test [36].


#### 4.1.7. Ballast Box

The ballast box apparatus is a steel box in 70 cm × 30 cm × 45 cm dimensions which can determine the lateral displacement, lateral and vertical strain, resilient modulus, permanent ballast deformation of the ballast layer according to Figure19. As depicted, the sample vertical displacements are calculated using 10 cm LVDT that is placed on a loading piston in 22 cm × 22 cm dimensions. Also, these dimensions cannot cover the actual sleeper dimensions, thus the outcomes of the experiment can be supposed as a representative of an actual condition of a ballasted track. Also, this apparatus has been used in Asian and Oceania countries such as Iran, China, India, and Australia.Figure 19
(a) Ballast box apparatus, (b) and (c) ballast box dimensions, and (d) LVDT location [37].


### 4.2. Railway Track Panel

Various laboratory apparatuses have been introduced to perform static and dynamic tests on the ballast and subballast layers. A major weakness of these tests was the lack of examination of other systems involved in the railway tracks, including rails, sleepers, and fasteners. Also, the railway track was not fully tested and only the ballast or subballast layers were under loading. Therefore, it is important to check the railway track with all the details involved. Now there are two conditions to test. The first model is to perform static and dynamic field tests in the conditions of train passing through the railway system, which is not one of the objectives of this study, and the second model is to simulate and design a railway track at the laboratory site with real or small scale, each of which is described in the following.


#### 4.2.1. Full Scale

In this section, all real-scale (1 : 1) railway track tests are presented, including fatigue test, railway test facility, and longitudinal and lateral resistance tests of the track, each of which is introduced. Note that these tests can be performed on a fixed railway panel. In other words, if there is a railway track with a scale of 1 : 1 and 3 to 5 sleepers, lateral, longitudinal, etc. tests can be conducted.(1) Fatigue Test. This apparatus is used to determine the fatigue life, displacement, stiffness, and probability of failure of railway tracks affected by quasi-static loading. The full scale reinforced concrete slabs were experimented using a 3-point bending test, namely, they were placed under a dynamic servo-hydraulic actuator, over 2 rubber supports with a length 47 cm and height 8.7 cm, enough to preclude the contact of slab and floor within this experiment. The density and elastic modulus of rubber were 1203 kg/m3 and 2.12 MPa, respectively. According to Figure 20, the loading was exerted on the middle section of the slab, throughout the rail and a specific instrument simulating the contact of wheel-rail of the train. The experiment process includes quasi-static loading test, before/after the fatigue test. Within the quasi-static test, strains, displacement, and loads were controlled in slab and rails. 4 LVDT were used in the slab middle section, 2 at both sides, for calculating the displacement of slab and actuator according to the laboratory floor.Figure 20
Fatigue test [38].(2) Railway Test Facility. The railway test facility is placed in a concrete pit in a dimension 410 cm (length) × 210 cm (width) × 190 cm (depth), which is shown in Figure 21. In this test, the ballast is housed and compacted in 10 cm layers by the same plate vibrator to 30 cm depth. 3 concrete sleepers are embedded in the ballast. Loadings are transferred from 3 servo-hydraulic actuators into the sleepers using the spreader beams (3 steel beams on the top of sleepers) placed on rollers on rail seatings. Each actuator has a built-in vertical displacement transducer to record the displacement. Besides the vertical displacement transducer in the middle actuator, the middle sleeper displacement is also calculated through 2 LVDT, at both middle sleeper ends to double examine the reading of settlement from the middle actuator. Volumetric strain, resilient modulus and permanent axial strain are the achieved parameters of this apparatus.Figure 21
The railway test facility [39].(3) Track Longitudinal Resistance Test. Figures (22) and (23) show the schematic and laboratory sample of track longitudinal resistance, respectively. This apparatus is used to evaluate the longitudinal resistance of the railway track on the laboratory scale (1 : 1). As can be seen, this apparatus includes a hydraulic loading Jack to simulate the dynamic sweep movement with a 300 kN control lever, one sensor unit, one data logger and other gadgets and measurement stuff. In this test, one dynamic sweep with the distinction loading system is applied to the aggregates of the ballast layer through the sleeper and rail fastening system and the effect of dynamic cyclic distribution is studied on the longitudinal resistance of ballast. The data logger gathers the information automatically through the computer connection and it processes the data related to the movement and applied forces and shows as output units. To record the required data during the test, two load cells and 6 LVDTs are needed. Therefore, in addition to longitudinal resistance, this apparatus can be used to determine the other results, such as track longitudinal stiffness and longitudinal displacement.Figure 22
Loading frame system [40].Figure 23
Track longitudinal resistance [40].(4) Track Lateral Resistance Test. For measuring the lateral resistance of railway track in the laboratory scale, there are three test techniques, including lateral track panel loading test (LTPT), single tie push test (STPT), and pendulum loading test apparatus, which are shown in Figures 24–26, respectively. Among them, STPT is the popular standard test to examine the single sleeper lateral resistance. This apparatus contains 3 principal sections, such as a transducer for calculating displacements, a loading Jack for imposing lateral forces, and a processor for collecting displacements and force amounts. The railway fastening systems of the middle sleeper were set free, and the hydraulic Jack and LVDTs were placed on the released sleeper. The Jack pulls the sleeper back from its support by enforcing the rail web. On another side of the sleeper, the displacement gauge stand is placed on the welded beam. This gauge calculates displacements while the sleeper moves. But on the contrary of STPT, LTPT apparatus operates the lateral loading on the railway track with three or five sleepers to evaluate the lateral resistance of the sleepers group, which is the advantage of this apparatus in proportion to STPT. However, the performance and operation of LTPT are similar to STPT, and both of them act statically. A load-cell is placed between the hydraulic cylinder and the rail to calculate the forces. On another side, the sleeper displacements are measured by separate LVDTs.Figure 24
The LTPT setup [41]: (a) hydraulic location. (b) LVDT position. (c) LVDT connection to sleepers.Figure 25
The STPT setup [42].Figure 26
Pendulum loading test apparatus [43].The lateral resistance of sleepers is the key parameter that guarantees the railway track lateral resistance and maintenance of geometric. The disadvantage of LTPT and STPT is that they only evaluate the lateral resistance statically and the railway track dynamic behavior is not investigated. To solve this problem, the pendulum loading test apparatus is suggested. This apparatus investigates the railway track lateral resistance dynamically. The other point of this apparatus is to investigate the interaction between the sleeper and the ballast regardless of rail.


#### 4.2.2. Small Scale

Unlike previous models with a scale of 1 : 1, this section presents a railway track with smaller scales of 1 : 3 to 1 : 8.5. First, a single-sleeper track is described in which vertical and lateral loads are applied to a 1 : 3 scaled railway track with one sleeper. The weakness of this apparatus is the lack of longitudinal load simulation, which has a significant role in track maintenance. Then a 1 : 3 scaled railway track with 3 sleepers is presented, which longitudinal loading is not provided again in this section, and finally, a 1 : 5 scaled railway track is presented which the lateral resistance of railway track is measured under the influence of horizontal, lateral and vertical loads and the effects of vibration on the ballast can even be evaluated.(1) Single Sleeper Track. The single sleeper track test is shown in Figure 27. As illustrated, this apparatus includes the whole railway track with a single sleeper which is built with a scale of 1 : 3. The significant point is that with this apparatus, the characteristics of various types of sleeper and rail and different types of ballast material can be measured affected by static loading. The test bay comprises 2 plates in vertical sides with 500 cm length and 65 cm height, built with heavy steel parts and panels. These are placed at a 65 cm distance apart, related to a usual sleeper space, by steel ties at the base and other different locations. The test bay is intended to maintain situations as near as executable to plane strain. Wooden panels are placed on inside walls of the test bay and a double plastic sheeting layer was located between the sidewalls and the ballast in an effort for minimizing the friction of side interface. A soft board of a total thickness 2 cm is located at the bay bottom to illustrate a somewhat compressible subgrade and establish a frictional contact. The ballast is embedded in a minimum depth of 30 cm. A loading beam across the railheads transferred loads from the hydraulic actuator to the rail. Moment loadings would be exerted by changing the vertical hydraulic actuator eccentricity.Figure 27
(a) Schematic view, and (b) image of laboratory sleeper test [25].
(a)(b)(2) Ballast Railway Track with Three Sleeper. This apparatus is a reduced scale test with 3 sleepers to examine the dynamic behaviour and the ballasted track displacement, as shown in Figure 28. This test presents results such as settlements, pressures, accelerations and displacements that permit us to further understand the dynamic behaviour of a portion of ballasted railway tracks at a reduced-scale and to examine the displacement against the number of loading cycles. Also, it can determine depth settlement, cumulative deformation, and permanent axial deformation. In addition, the effect of wetting-drying cycles can be considered. The sleepers are loaded using 3 hydraulic jacks which present signals that have an M shape.Figure 28
Ballast railway track with three sleeper test [25].The test allows examining the ballasted railway track response because of the passage of high speed train. From bottom to top, the portion contains a layer of 60 cm height soil compacted in 3 layers with 20 cm height, a layer of compacted microballast (10 cm equivalent to 30 cm in actual scales) on which 3 sleepers are located based on the positions of hydraulic jacks, steel-created wedges placed on each sleeper for transferring loadings exerted by the jacks into the blocks and a layer of uncompacted microballast that is provided up to the blocks’ upper surface (8 cm). The distance of 2 sleepers is 20 cm at a reduced-scale, related to the actual-scale distance 60 cm. It is supported by a solid mass which comprises from bottom to top, a sand layer (250 cm), a reinforced concrete layer (100 cm) and a horizontal steel plate with 7 cm thick. The solid mass contained various substances is regarded as an infinite half-space [44]. The ballast layers are confined by inclined planes on 2 opposite sides which illustrate free edges. The other sides are confined by vertical steel plates (5 cm) that represent the end and the beginning of the portion. The hydraulic jacks are placed on a steel frame that is bolted on a horizontal steel plate.(3) Scaled Railway Track Models. As previously mentioned, the railway track with the laboratory scale provides the comprehensive investigation feasibility of it affected by the vertical, horizontal, and lateral displacement and the significant point is that the problems caused by field tests are also minimized. The smaller these tracks are made, the lower the construction and maintenance costs will be. After reviewing various researches, it was found that models with a scale of 1 : 5 and 1 : 8.5 have been made, which are smaller than previous scaled tracks and despite being small, it can be investigated the large area of ballast track and more sleepers. Figures 29 and 30 show the railway tracks with a scale of 1 : 5 in a rigid frame and without a frame, respectively. In model 1, the researcher investigates the lateral, vertical, and horizontal displacement affected by the horizontal loading. Also, it can be evaluated the effect of sleepers spacing, the effect of various types of sleepers (wooden, concrete, steel), and different types of ballast materials, and simulated the laboratory condition and freeze-dry cycles of winter seasons. But in model 2, it can be measured the sound radiation sound pressure level affected by vertical and lateral displacement. In general, the nature of both apparatuses is the same and the only difference is the rigid frame of model 1 that provides a platform for loading.Figure 29
1 : 5 scaled railway track in the rigid frame with five sleepers embedded in road beds [45].
(a)(b)(c)Figure 30
1 : 5 scaled railway track without the rigid frame with several sleepers [46].But there is a smaller model according to Figure31, which is sorted in both categories of scaled railway track and roller rig. In this model, it can be measured rolling noise and rail vibration, frequency spectra, and weighted sound pressure level. In addition, the effect of various fastening and sleepers, the sleepers’ space, and interaction between the track and locomotive can be achieved.Figure 31
The test rig for scale-model test with scale ratio of 1 : 8.5. (a) Overall view. (b) Measurement section [47].
(a)(b)


### 4.3. Roller Rig

One of the first studies to evaluate the flow performance of locomotives using a roller rig can be referred to the United Kingdom in 1904, which were continued with expanding of the railway system and related technologies in other countries of Asia, Europe, and American. This apparatus can be used in various forms and scales to investigate the railway vehicle dynamic and wheel-rail contact studies, which are described as follow. Also, the design of the roller rig is different according to the laboratory purposes. Among the parameters affecting the design of roller rigs, scale, test specimen and design concept can be mentioned [48, 49].The scale divides roller rig in 2 major classes: scaled and full-scale. Scaled designs present various advantages namely the easier implementation of parameter changes, lower space requirement, lower energy consumption, lower operation cost and lower investments. Nevertheless, just scaled models could be experimented, instead of whole vehicles or standard components. Also, interpreting the outcomes of the scaled test is not straight and is dependent on the selected scaling strategy [50].Based on the experimented specimens, the full vehicle test rigs, bogie, wheelset and single wheel are differentiated. The single wheelset setup remarkably enhances the system complication over the single wheel designs, however, it presents various advantages, particularly the capability to examine the interactions related to the suspension setup and dynamics of a single axle. Testing an assembled a whole vehicle or bogie notably enhances the complexity of the system over both the wheelset and single-wheel design, however, it potentially presents various additional benefits since vehicle rigs and bogie can be utilized to study the railway vehicle as a complete system.Many roller rig design concepts according to Figure32 are known that the most usual of these concepts is the vertical plane roller concept (Figure 32(a)). Other concepts including perpendicular roller (Figure 32(b)), internal roller (Figure 32(c)), or oscillating rail (Figure 32(d)) are commonly provided for special goal and the use of such designs is entirely uncommon in comparison with the standard vertical plane roller concept [51].Figure 32
Roller rig design concepts. (a) Vertical plane roller. (b) Perpendicular roller. (c) Internal roller. (d) Oscillating rail [51].
(a)(b)(c)(d)


#### 4.3.1. Roller Rig for Railway Vehicle Dynamic Studies

Since single wheel rigs cannot be utilized to hunt researches, a single wheelset rig is minimum rig configuration in order to study a railway vehicle dynamic. However, the most usual rigs in this field are the full scale vertical roller rig for test the whole vehicles. The first apparatus of this type was utilized in the start of the twentieth century. Within those first experiments, the increasing intensity of lateral vibration when enhancing the peripheral speed of roller, known as hunting motion later, was identified. This event was regarded so considerable based on enhancing operation speed on the track and using roller rigs for railway vehicle running dynamic examination have started. The significance of these rigs has raised with high-speed rail developments. The most advanced rigs of this type, as are shown in Figure33, permit to be setup for various vehicle dimensions, loadings and wheel set gauge, and simulate vehicle running up to speed 500 km/h for broad ranges of track situations, including curved track and track irregularities. Since full bogie or vehicle can experiment under conditions same as a real track, experiments on this type of roller rigs came closer to reality. However, the construction and operating costs of these apparatuses are so great. Today, in the field of vehicle developments, the role of full-scale roller rigs is being exchanged by much more inexpensive computer-based simulations. Nonetheless, this kind of roller rigs still has a significant role in countries that begun to extend their own fleet of high speed vehicles [51].Figure 33
Two samples of full scaled roller dig in the world [48].Another test rig type that is used to study railway vehicle dynamics are scaled roller rigs, commonly designed for 1/5 to 1/3 scaled models of two axle bogies. Among significant disadvantages of a scaled system is the impossibility to test standard components directly and troubles with the interpretations of scaled-test outcomes in a full scale world. Using the scaled roller rigs in the development procedure of a specific vehicle is therefore considerably limited. Scaled roller rigs are generally constructed in research institutions or universities for developments and the test of entirely novel running gear concepts, namely, active-controlled drives of independently rotating wheels (Figure34(a)) [52], active-controlled wheel set steering (Figure 34(b)) [53, 54] or inverse thread wheel sets [51]. Moreover, a particular class of scaled experiment is testing on scaled tracks. In case of scaling of 1/5 or further, the whole track containing a straight and one or two curved parts can be located in a lab (Figure 35).Figure 34
(a) Roller rig with scale 1/5. (b) Roller rig with scale 1/3.5 [18].
(a)(b)Figure 35
(a) Steering railway bogies with scale 1/10 [55]. (b) Active steering vehicle with scale 1/5 [56].
(a)(b)


#### 4.3.2. Roller Rigs for Wheel-Rail Contact Researches

Study of phenomena of wheel-rail contacts needs an accurate position of a roller and wheel, exact control of wheel and roller speed and accurate control of wheel loadings. The complex designs of a bogie enhance the complication to meet the needs. So roller rigs for examination the contact of wheel and rail are commonly in a single wheel form on roller or wheel set on rollers. Both full scale and scaled rigs are utilized (Figure36). Moreover, study of phenomena of wheel-rail contacts needs simulations of contact point factors as close as practicable to the track situations. The finite diameter of roller results in unavoidable faults of wheel-rail and wheel-roller contacts. In addition to standard vertical roller rigs, horizontal roller or inner roller concepts are also utilized (Figure 37(a)) since such designs permit errors decrease by enhancing the diameters of roller and keeping the overall rig diameters specified limits at the same time. A particular kind of test rig type is oscillating rail rig (Figure 37(b)).Figure 36
(a) Roller rig with small scale [57], (b) full-scaled roller rig (b) [58].
(a)(b)Figure 37
(a) Wheel-on-roller test rig with scale 1/5 [59], (b) full scale of wheel-on-roller test rig [60].
(a)(b)


## 4.1. Ballast and Subgrade Tests

The infrastructure and ballast layer have a significant role in loading and resistance of a railway track, and therefore, researchers follow the methods to enhance the performance and loading of railway tracks through the reduction of lateral movement, permanent deformation, and settlement of railway system. These include adding crumbs and sand to the ballast and placing the geogrid in the ballast mixture. To evaluate and determine the loading amount of the ballast layer, it should be fulfilled various tests that, according to purposes and laboratory investment cost, different ballast apparatuses with the various scales have been built, which are described as follows.


### 4.1.1. Large and Small Scale Triaxial Tests

This apparatus can be used to determine the permanent deformation, degradation behavior during monotonic and cyclic vertical loadings, and permanent axial strain, which includes full scale and small scale. Figure13(a) illustrates the full-scale test setup. A railway track section is constructed with ballast, subballast, and subgrade containing crushed rocks and confined with a rigid experiment box with 1.5 m wide and 3 m long, which all are rested on a concrete floor. Under the upper layer of ballast, the support layers are neither changed nor removed within the complete experiment. This test apparatus has been built in Iran, Norway, and the United Kingdom on the full scale. Also, the small scale of this test is shown in Figure 13(b), which usually are built on a 1 : 5 scale. Different countries such as United States, Canada, Brazil, United Kingdom, Australia, and France use this method to evaluate the ballast layer. This apparatus uses a hydraulic mechanism to simulate loading on the ballast layer in the laboratory scale and its load frame is able to apply both cyclic and monotonic loads to a specimen. The maximum force that is applicable is 100 kN that is equivalent to a 5659 kPa deviatoric stress on the specimen (15 cm diameter). The axial loading is exerted on the specimen by the bottom movement against the stationary loading cell at the top of the specimen. The confinement method used was a vacuum system that is sufficient for materials placed relatively close to the surface, and therefore under relatively low confinement. The maximum confinement available using vacuum is 95 kPa (close to 1 atm).Figure 13
Small-scale cylindrical triaxial apparatus: (a) full-scaled [27], (b) small-scaled [28].
(a)(b)


### 4.1.2. Sleeper and Ballast Track

The experiment model a 1/3 size sleeper end being pushed gently to a shoulder constructed with a scaled ballast. The displacement is controlled using linear variable displacement transducer (LVDT) and optically, and the resistance on the sleeper end is examined using a loading cell. The sample ballast shoulder is confined between vertical wooden borders located appropriately beyond the expected amount of failure mechanisms, as illustrated in the overall view of the experiment setup displayed in Figure14. Therefore, the testing apparatus boundaries have not any effect on outcomes. The ballast bed extends to 11 cm depth under the sample sleeper end bottom, related to 33 cm at full-scale. A rough sandpaper mat at the base of ballasts prevents ballast materials from sliding along the junction with the wooden surface on which experiments are performed.Figure 14
The sleeper and ballast track apparatus [29].


### 4.1.3. Moving-Wheel Loading

This test has been used to investigate the performance of ballast layer and subgrade on two scales: full and small (1 : 5), which the full one is shown according to Figure15. Also, it can be conducted to examine the elastic deformation, dynamic soil pressure, vibration speed and cumulative settlements bellow simulated dynamic train loading. In full-scale condition, the subgrade constituted by unbound granular substances has a 2.7 × 5 × 13.1 m dimensions in depth, width and length, respectively. The dynamic train loadings are simulated by the set of phase difference of contiguous actuators (8 actuators in tests) and the speed of simulated maximum train is 360 km/h [30, 31]. But in small-scale condition, all dimensions were changed into 1 : 5 scale.Figure 15
The moving-wheel loading test [32].


### 4.1.4. Stone Blowing

The apparatus presented in Figure16 is used to determine the settlement amount and the ballast layer behavior affected by repeating loading. A rubber mat with the 1 cm thickness and 5000 kPa reported elastic modulus is glued to this box bottom for simulating the actions of track subgrade, on which the experiment substance is placed. Substances with a field size are utilized with the main ballast bed composed with 5 cm single size angular medium gravels and 3 smaller single size gravels that are utilized for the upper stone layer. The test bed is loaded using a sample reinforced concrete sleeper with dimensions 60 cm by 10 cm by 10 cm in length, width and deep, respectively, which is constrained by 2 arms bolted upon the sleeper and connected to the apparatus sides. These arms are articulated to preclude movements other than vertical direction and simulate the constraint of rail and sleeper fastenings in actual railway tracks. The sleeper is under the load of a hydraulic ram using a steel ball and seat arrangement on the sleeper top. This ram reacts against the top of a pair of steel beams that connectivity with it permits the ram to be angled in any direction from the vertical, however, in experiments presented here, only vertical loadings are applied. The hydraulic ram is fed using a high-capacity pump and is a servo-hydraulic computer-controlled valve for operating in various modes. A sine wave loading-unloading cycle is used in the experiments. Integrated with the hydraulic ram are transducers to calculate the loading on the sleeper and sleeper displacements corresponding to the start situation. The sample sleeper rigidity is such that no bending happens under loadings.Figure 16
Stone blowing [33].


### 4.1.5. Direct Shear Test Machine

Inasmuch as ballast substances are constituted with coarse materials and aggregates, a large-scale direct shear test machine can be applied to evaluate the shear stress and horizontal displacement of railway ballast materials. A sample of this machine was fabricated and designed at Iran University of Science and Technology, in which there are two shear boxes, the upper and the lower. The lower box has 440× 540 mm dimensions in width and length, respectively. The upper shear box has a square form and 440 mm length. Both boxes have the same height of 180 mm. Figure 17(a) illustrates the side view of the machine. Also, Figure 17(b) indicates the top and bottom shear boxes. Ballast specimens are placed in 2 layers in these shear boxes. The ballast is compacted in the layers similar to the field by the use of a hand tamper to achieve the 17.5 kN/m3 dry density [34]. Based on ASTM D3080, direct shear tests are performed. For all tests, the upper shear box movements are constrained in longitudinal and lateral. The lower shear box is moved in the longitudinal direction at a constant rate [34]. Vertical stress is exerted on the ballast surface in the upper shear box using a Jack which transfers the normal forces to a steel rigid plate placed on the ballast top. The vertical stress remains constant in the tests. 50, 100, 150, and 200 kPa normal pressures are exerted to the specimens within the tests. By the use of LVDT and loading cells, the horizontal and vertical displacements and the exerted forces are concurrently calculated. All sensors are connected to a data logger that records information in the tests.Figure 17
Direct shear test machine [35]: (a) overall view of the machine, (b) dimension of the boxes.
(a)(b)


### 4.1.6. Plate Load Test (PLT)

This apparatus is used to evaluate stiffness, permanent deformation, settlement of the ballast layer of the railway. In order to conduct PLT, ballast substances are placed in a large steel chamber, which has dimensions of 1200, 1200, and 1000 mm in length, width, and depth, respectively, as illustrated in Figure18. For PLT, 3 clean ballast layers should be placed in the chamber, the equal thickness of which is 300 mm that is close to the depth of the ballast layer in the railway field. The 17.5 kN/m3 density is achieved for the dry density of clean ballasts. A hydraulic Jack with 30 ton capacity is usually utilized in order to apply loads to the plate. Moreover, a rigid frame fixed to the laboratory floor strongly is utilized for supporting the Jack for vertical loads. The chamber is mounted bellow the rigid frame beam. The data recording setup and steel chamber are other related apparatuses. A rigid steel plate in a square form is utilized as the loading plate that is placed on the ballast surface, the dimensions of which are 15 × 30 cm in thickness and width, respectively.Figure 18
Plate load test [36].


### 4.1.7. Ballast Box

The ballast box apparatus is a steel box in 70 cm × 30 cm × 45 cm dimensions which can determine the lateral displacement, lateral and vertical strain, resilient modulus, permanent ballast deformation of the ballast layer according to Figure19. As depicted, the sample vertical displacements are calculated using 10 cm LVDT that is placed on a loading piston in 22 cm × 22 cm dimensions. Also, these dimensions cannot cover the actual sleeper dimensions, thus the outcomes of the experiment can be supposed as a representative of an actual condition of a ballasted track. Also, this apparatus has been used in Asian and Oceania countries such as Iran, China, India, and Australia.Figure 19
(a) Ballast box apparatus, (b) and (c) ballast box dimensions, and (d) LVDT location [37].


## 4.1.1. Large and Small Scale Triaxial Tests

This apparatus can be used to determine the permanent deformation, degradation behavior during monotonic and cyclic vertical loadings, and permanent axial strain, which includes full scale and small scale. Figure13(a) illustrates the full-scale test setup. A railway track section is constructed with ballast, subballast, and subgrade containing crushed rocks and confined with a rigid experiment box with 1.5 m wide and 3 m long, which all are rested on a concrete floor. Under the upper layer of ballast, the support layers are neither changed nor removed within the complete experiment. This test apparatus has been built in Iran, Norway, and the United Kingdom on the full scale. Also, the small scale of this test is shown in Figure 13(b), which usually are built on a 1 : 5 scale. Different countries such as United States, Canada, Brazil, United Kingdom, Australia, and France use this method to evaluate the ballast layer. This apparatus uses a hydraulic mechanism to simulate loading on the ballast layer in the laboratory scale and its load frame is able to apply both cyclic and monotonic loads to a specimen. The maximum force that is applicable is 100 kN that is equivalent to a 5659 kPa deviatoric stress on the specimen (15 cm diameter). The axial loading is exerted on the specimen by the bottom movement against the stationary loading cell at the top of the specimen. The confinement method used was a vacuum system that is sufficient for materials placed relatively close to the surface, and therefore under relatively low confinement. The maximum confinement available using vacuum is 95 kPa (close to 1 atm).Figure 13
Small-scale cylindrical triaxial apparatus: (a) full-scaled [27], (b) small-scaled [28].
(a)(b)


## 4.1.2. Sleeper and Ballast Track

The experiment model a 1/3 size sleeper end being pushed gently to a shoulder constructed with a scaled ballast. The displacement is controlled using linear variable displacement transducer (LVDT) and optically, and the resistance on the sleeper end is examined using a loading cell. The sample ballast shoulder is confined between vertical wooden borders located appropriately beyond the expected amount of failure mechanisms, as illustrated in the overall view of the experiment setup displayed in Figure14. Therefore, the testing apparatus boundaries have not any effect on outcomes. The ballast bed extends to 11 cm depth under the sample sleeper end bottom, related to 33 cm at full-scale. A rough sandpaper mat at the base of ballasts prevents ballast materials from sliding along the junction with the wooden surface on which experiments are performed.Figure 14
The sleeper and ballast track apparatus [29].


## 4.1.3. Moving-Wheel Loading

This test has been used to investigate the performance of ballast layer and subgrade on two scales: full and small (1 : 5), which the full one is shown according to Figure15. Also, it can be conducted to examine the elastic deformation, dynamic soil pressure, vibration speed and cumulative settlements bellow simulated dynamic train loading. In full-scale condition, the subgrade constituted by unbound granular substances has a 2.7 × 5 × 13.1 m dimensions in depth, width and length, respectively. The dynamic train loadings are simulated by the set of phase difference of contiguous actuators (8 actuators in tests) and the speed of simulated maximum train is 360 km/h [30, 31]. But in small-scale condition, all dimensions were changed into 1 : 5 scale.Figure 15
The moving-wheel loading test [32].


## 4.1.4. Stone Blowing

The apparatus presented in Figure16 is used to determine the settlement amount and the ballast layer behavior affected by repeating loading. A rubber mat with the 1 cm thickness and 5000 kPa reported elastic modulus is glued to this box bottom for simulating the actions of track subgrade, on which the experiment substance is placed. Substances with a field size are utilized with the main ballast bed composed with 5 cm single size angular medium gravels and 3 smaller single size gravels that are utilized for the upper stone layer. The test bed is loaded using a sample reinforced concrete sleeper with dimensions 60 cm by 10 cm by 10 cm in length, width and deep, respectively, which is constrained by 2 arms bolted upon the sleeper and connected to the apparatus sides. These arms are articulated to preclude movements other than vertical direction and simulate the constraint of rail and sleeper fastenings in actual railway tracks. The sleeper is under the load of a hydraulic ram using a steel ball and seat arrangement on the sleeper top. This ram reacts against the top of a pair of steel beams that connectivity with it permits the ram to be angled in any direction from the vertical, however, in experiments presented here, only vertical loadings are applied. The hydraulic ram is fed using a high-capacity pump and is a servo-hydraulic computer-controlled valve for operating in various modes. A sine wave loading-unloading cycle is used in the experiments. Integrated with the hydraulic ram are transducers to calculate the loading on the sleeper and sleeper displacements corresponding to the start situation. The sample sleeper rigidity is such that no bending happens under loadings.Figure 16
Stone blowing [33].


## 4.1.5. Direct Shear Test Machine

Inasmuch as ballast substances are constituted with coarse materials and aggregates, a large-scale direct shear test machine can be applied to evaluate the shear stress and horizontal displacement of railway ballast materials. A sample of this machine was fabricated and designed at Iran University of Science and Technology, in which there are two shear boxes, the upper and the lower. The lower box has 440× 540 mm dimensions in width and length, respectively. The upper shear box has a square form and 440 mm length. Both boxes have the same height of 180 mm. Figure 17(a) illustrates the side view of the machine. Also, Figure 17(b) indicates the top and bottom shear boxes. Ballast specimens are placed in 2 layers in these shear boxes. The ballast is compacted in the layers similar to the field by the use of a hand tamper to achieve the 17.5 kN/m3 dry density [34]. Based on ASTM D3080, direct shear tests are performed. For all tests, the upper shear box movements are constrained in longitudinal and lateral. The lower shear box is moved in the longitudinal direction at a constant rate [34]. Vertical stress is exerted on the ballast surface in the upper shear box using a Jack which transfers the normal forces to a steel rigid plate placed on the ballast top. The vertical stress remains constant in the tests. 50, 100, 150, and 200 kPa normal pressures are exerted to the specimens within the tests. By the use of LVDT and loading cells, the horizontal and vertical displacements and the exerted forces are concurrently calculated. All sensors are connected to a data logger that records information in the tests.Figure 17
Direct shear test machine [35]: (a) overall view of the machine, (b) dimension of the boxes.
(a)(b)


## 4.1.6. Plate Load Test (PLT)

This apparatus is used to evaluate stiffness, permanent deformation, settlement of the ballast layer of the railway. In order to conduct PLT, ballast substances are placed in a large steel chamber, which has dimensions of 1200, 1200, and 1000 mm in length, width, and depth, respectively, as illustrated in Figure18. For PLT, 3 clean ballast layers should be placed in the chamber, the equal thickness of which is 300 mm that is close to the depth of the ballast layer in the railway field. The 17.5 kN/m3 density is achieved for the dry density of clean ballasts. A hydraulic Jack with 30 ton capacity is usually utilized in order to apply loads to the plate. Moreover, a rigid frame fixed to the laboratory floor strongly is utilized for supporting the Jack for vertical loads. The chamber is mounted bellow the rigid frame beam. The data recording setup and steel chamber are other related apparatuses. A rigid steel plate in a square form is utilized as the loading plate that is placed on the ballast surface, the dimensions of which are 15 × 30 cm in thickness and width, respectively.Figure 18
Plate load test [36].


## 4.1.7. Ballast Box

The ballast box apparatus is a steel box in 70 cm × 30 cm × 45 cm dimensions which can determine the lateral displacement, lateral and vertical strain, resilient modulus, permanent ballast deformation of the ballast layer according to Figure19. As depicted, the sample vertical displacements are calculated using 10 cm LVDT that is placed on a loading piston in 22 cm × 22 cm dimensions. Also, these dimensions cannot cover the actual sleeper dimensions, thus the outcomes of the experiment can be supposed as a representative of an actual condition of a ballasted track. Also, this apparatus has been used in Asian and Oceania countries such as Iran, China, India, and Australia.Figure 19
(a) Ballast box apparatus, (b) and (c) ballast box dimensions, and (d) LVDT location [37].


## 4.2. Railway Track Panel

Various laboratory apparatuses have been introduced to perform static and dynamic tests on the ballast and subballast layers. A major weakness of these tests was the lack of examination of other systems involved in the railway tracks, including rails, sleepers, and fasteners. Also, the railway track was not fully tested and only the ballast or subballast layers were under loading. Therefore, it is important to check the railway track with all the details involved. Now there are two conditions to test. The first model is to perform static and dynamic field tests in the conditions of train passing through the railway system, which is not one of the objectives of this study, and the second model is to simulate and design a railway track at the laboratory site with real or small scale, each of which is described in the following.


### 4.2.1. Full Scale

In this section, all real-scale (1 : 1) railway track tests are presented, including fatigue test, railway test facility, and longitudinal and lateral resistance tests of the track, each of which is introduced. Note that these tests can be performed on a fixed railway panel. In other words, if there is a railway track with a scale of 1 : 1 and 3 to 5 sleepers, lateral, longitudinal, etc. tests can be conducted.(1) Fatigue Test. This apparatus is used to determine the fatigue life, displacement, stiffness, and probability of failure of railway tracks affected by quasi-static loading. The full scale reinforced concrete slabs were experimented using a 3-point bending test, namely, they were placed under a dynamic servo-hydraulic actuator, over 2 rubber supports with a length 47 cm and height 8.7 cm, enough to preclude the contact of slab and floor within this experiment. The density and elastic modulus of rubber were 1203 kg/m3 and 2.12 MPa, respectively. According to Figure 20, the loading was exerted on the middle section of the slab, throughout the rail and a specific instrument simulating the contact of wheel-rail of the train. The experiment process includes quasi-static loading test, before/after the fatigue test. Within the quasi-static test, strains, displacement, and loads were controlled in slab and rails. 4 LVDT were used in the slab middle section, 2 at both sides, for calculating the displacement of slab and actuator according to the laboratory floor.Figure 20
Fatigue test [38].(2) Railway Test Facility. The railway test facility is placed in a concrete pit in a dimension 410 cm (length) × 210 cm (width) × 190 cm (depth), which is shown in Figure 21. In this test, the ballast is housed and compacted in 10 cm layers by the same plate vibrator to 30 cm depth. 3 concrete sleepers are embedded in the ballast. Loadings are transferred from 3 servo-hydraulic actuators into the sleepers using the spreader beams (3 steel beams on the top of sleepers) placed on rollers on rail seatings. Each actuator has a built-in vertical displacement transducer to record the displacement. Besides the vertical displacement transducer in the middle actuator, the middle sleeper displacement is also calculated through 2 LVDT, at both middle sleeper ends to double examine the reading of settlement from the middle actuator. Volumetric strain, resilient modulus and permanent axial strain are the achieved parameters of this apparatus.Figure 21
The railway test facility [39].(3) Track Longitudinal Resistance Test. Figures (22) and (23) show the schematic and laboratory sample of track longitudinal resistance, respectively. This apparatus is used to evaluate the longitudinal resistance of the railway track on the laboratory scale (1 : 1). As can be seen, this apparatus includes a hydraulic loading Jack to simulate the dynamic sweep movement with a 300 kN control lever, one sensor unit, one data logger and other gadgets and measurement stuff. In this test, one dynamic sweep with the distinction loading system is applied to the aggregates of the ballast layer through the sleeper and rail fastening system and the effect of dynamic cyclic distribution is studied on the longitudinal resistance of ballast. The data logger gathers the information automatically through the computer connection and it processes the data related to the movement and applied forces and shows as output units. To record the required data during the test, two load cells and 6 LVDTs are needed. Therefore, in addition to longitudinal resistance, this apparatus can be used to determine the other results, such as track longitudinal stiffness and longitudinal displacement.Figure 22
Loading frame system [40].Figure 23
Track longitudinal resistance [40].(4) Track Lateral Resistance Test. For measuring the lateral resistance of railway track in the laboratory scale, there are three test techniques, including lateral track panel loading test (LTPT), single tie push test (STPT), and pendulum loading test apparatus, which are shown in Figures 24–26, respectively. Among them, STPT is the popular standard test to examine the single sleeper lateral resistance. This apparatus contains 3 principal sections, such as a transducer for calculating displacements, a loading Jack for imposing lateral forces, and a processor for collecting displacements and force amounts. The railway fastening systems of the middle sleeper were set free, and the hydraulic Jack and LVDTs were placed on the released sleeper. The Jack pulls the sleeper back from its support by enforcing the rail web. On another side of the sleeper, the displacement gauge stand is placed on the welded beam. This gauge calculates displacements while the sleeper moves. But on the contrary of STPT, LTPT apparatus operates the lateral loading on the railway track with three or five sleepers to evaluate the lateral resistance of the sleepers group, which is the advantage of this apparatus in proportion to STPT. However, the performance and operation of LTPT are similar to STPT, and both of them act statically. A load-cell is placed between the hydraulic cylinder and the rail to calculate the forces. On another side, the sleeper displacements are measured by separate LVDTs.Figure 24
The LTPT setup [41]: (a) hydraulic location. (b) LVDT position. (c) LVDT connection to sleepers.Figure 25
The STPT setup [42].Figure 26
Pendulum loading test apparatus [43].The lateral resistance of sleepers is the key parameter that guarantees the railway track lateral resistance and maintenance of geometric. The disadvantage of LTPT and STPT is that they only evaluate the lateral resistance statically and the railway track dynamic behavior is not investigated. To solve this problem, the pendulum loading test apparatus is suggested. This apparatus investigates the railway track lateral resistance dynamically. The other point of this apparatus is to investigate the interaction between the sleeper and the ballast regardless of rail.


### 4.2.2. Small Scale

Unlike previous models with a scale of 1 : 1, this section presents a railway track with smaller scales of 1 : 3 to 1 : 8.5. First, a single-sleeper track is described in which vertical and lateral loads are applied to a 1 : 3 scaled railway track with one sleeper. The weakness of this apparatus is the lack of longitudinal load simulation, which has a significant role in track maintenance. Then a 1 : 3 scaled railway track with 3 sleepers is presented, which longitudinal loading is not provided again in this section, and finally, a 1 : 5 scaled railway track is presented which the lateral resistance of railway track is measured under the influence of horizontal, lateral and vertical loads and the effects of vibration on the ballast can even be evaluated.(1) Single Sleeper Track. The single sleeper track test is shown in Figure 27. As illustrated, this apparatus includes the whole railway track with a single sleeper which is built with a scale of 1 : 3. The significant point is that with this apparatus, the characteristics of various types of sleeper and rail and different types of ballast material can be measured affected by static loading. The test bay comprises 2 plates in vertical sides with 500 cm length and 65 cm height, built with heavy steel parts and panels. These are placed at a 65 cm distance apart, related to a usual sleeper space, by steel ties at the base and other different locations. The test bay is intended to maintain situations as near as executable to plane strain. Wooden panels are placed on inside walls of the test bay and a double plastic sheeting layer was located between the sidewalls and the ballast in an effort for minimizing the friction of side interface. A soft board of a total thickness 2 cm is located at the bay bottom to illustrate a somewhat compressible subgrade and establish a frictional contact. The ballast is embedded in a minimum depth of 30 cm. A loading beam across the railheads transferred loads from the hydraulic actuator to the rail. Moment loadings would be exerted by changing the vertical hydraulic actuator eccentricity.Figure 27
(a) Schematic view, and (b) image of laboratory sleeper test [25].
(a)(b)(2) Ballast Railway Track with Three Sleeper. This apparatus is a reduced scale test with 3 sleepers to examine the dynamic behaviour and the ballasted track displacement, as shown in Figure 28. This test presents results such as settlements, pressures, accelerations and displacements that permit us to further understand the dynamic behaviour of a portion of ballasted railway tracks at a reduced-scale and to examine the displacement against the number of loading cycles. Also, it can determine depth settlement, cumulative deformation, and permanent axial deformation. In addition, the effect of wetting-drying cycles can be considered. The sleepers are loaded using 3 hydraulic jacks which present signals that have an M shape.Figure 28
Ballast railway track with three sleeper test [25].The test allows examining the ballasted railway track response because of the passage of high speed train. From bottom to top, the portion contains a layer of 60 cm height soil compacted in 3 layers with 20 cm height, a layer of compacted microballast (10 cm equivalent to 30 cm in actual scales) on which 3 sleepers are located based on the positions of hydraulic jacks, steel-created wedges placed on each sleeper for transferring loadings exerted by the jacks into the blocks and a layer of uncompacted microballast that is provided up to the blocks’ upper surface (8 cm). The distance of 2 sleepers is 20 cm at a reduced-scale, related to the actual-scale distance 60 cm. It is supported by a solid mass which comprises from bottom to top, a sand layer (250 cm), a reinforced concrete layer (100 cm) and a horizontal steel plate with 7 cm thick. The solid mass contained various substances is regarded as an infinite half-space [44]. The ballast layers are confined by inclined planes on 2 opposite sides which illustrate free edges. The other sides are confined by vertical steel plates (5 cm) that represent the end and the beginning of the portion. The hydraulic jacks are placed on a steel frame that is bolted on a horizontal steel plate.(3) Scaled Railway Track Models. As previously mentioned, the railway track with the laboratory scale provides the comprehensive investigation feasibility of it affected by the vertical, horizontal, and lateral displacement and the significant point is that the problems caused by field tests are also minimized. The smaller these tracks are made, the lower the construction and maintenance costs will be. After reviewing various researches, it was found that models with a scale of 1 : 5 and 1 : 8.5 have been made, which are smaller than previous scaled tracks and despite being small, it can be investigated the large area of ballast track and more sleepers. Figures 29 and 30 show the railway tracks with a scale of 1 : 5 in a rigid frame and without a frame, respectively. In model 1, the researcher investigates the lateral, vertical, and horizontal displacement affected by the horizontal loading. Also, it can be evaluated the effect of sleepers spacing, the effect of various types of sleepers (wooden, concrete, steel), and different types of ballast materials, and simulated the laboratory condition and freeze-dry cycles of winter seasons. But in model 2, it can be measured the sound radiation sound pressure level affected by vertical and lateral displacement. In general, the nature of both apparatuses is the same and the only difference is the rigid frame of model 1 that provides a platform for loading.Figure 29
1 : 5 scaled railway track in the rigid frame with five sleepers embedded in road beds [45].
(a)(b)(c)Figure 30
1 : 5 scaled railway track without the rigid frame with several sleepers [46].But there is a smaller model according to Figure31, which is sorted in both categories of scaled railway track and roller rig. In this model, it can be measured rolling noise and rail vibration, frequency spectra, and weighted sound pressure level. In addition, the effect of various fastening and sleepers, the sleepers’ space, and interaction between the track and locomotive can be achieved.Figure 31
The test rig for scale-model test with scale ratio of 1 : 8.5. (a) Overall view. (b) Measurement section [47].
(a)(b)


## 4.2.1. Full Scale

In this section, all real-scale (1 : 1) railway track tests are presented, including fatigue test, railway test facility, and longitudinal and lateral resistance tests of the track, each of which is introduced. Note that these tests can be performed on a fixed railway panel. In other words, if there is a railway track with a scale of 1 : 1 and 3 to 5 sleepers, lateral, longitudinal, etc. tests can be conducted.(1) Fatigue Test. This apparatus is used to determine the fatigue life, displacement, stiffness, and probability of failure of railway tracks affected by quasi-static loading. The full scale reinforced concrete slabs were experimented using a 3-point bending test, namely, they were placed under a dynamic servo-hydraulic actuator, over 2 rubber supports with a length 47 cm and height 8.7 cm, enough to preclude the contact of slab and floor within this experiment. The density and elastic modulus of rubber were 1203 kg/m3 and 2.12 MPa, respectively. According to Figure 20, the loading was exerted on the middle section of the slab, throughout the rail and a specific instrument simulating the contact of wheel-rail of the train. The experiment process includes quasi-static loading test, before/after the fatigue test. Within the quasi-static test, strains, displacement, and loads were controlled in slab and rails. 4 LVDT were used in the slab middle section, 2 at both sides, for calculating the displacement of slab and actuator according to the laboratory floor.Figure 20
Fatigue test [38].(2) Railway Test Facility. The railway test facility is placed in a concrete pit in a dimension 410 cm (length) × 210 cm (width) × 190 cm (depth), which is shown in Figure 21. In this test, the ballast is housed and compacted in 10 cm layers by the same plate vibrator to 30 cm depth. 3 concrete sleepers are embedded in the ballast. Loadings are transferred from 3 servo-hydraulic actuators into the sleepers using the spreader beams (3 steel beams on the top of sleepers) placed on rollers on rail seatings. Each actuator has a built-in vertical displacement transducer to record the displacement. Besides the vertical displacement transducer in the middle actuator, the middle sleeper displacement is also calculated through 2 LVDT, at both middle sleeper ends to double examine the reading of settlement from the middle actuator. Volumetric strain, resilient modulus and permanent axial strain are the achieved parameters of this apparatus.Figure 21
The railway test facility [39].(3) Track Longitudinal Resistance Test. Figures (22) and (23) show the schematic and laboratory sample of track longitudinal resistance, respectively. This apparatus is used to evaluate the longitudinal resistance of the railway track on the laboratory scale (1 : 1). As can be seen, this apparatus includes a hydraulic loading Jack to simulate the dynamic sweep movement with a 300 kN control lever, one sensor unit, one data logger and other gadgets and measurement stuff. In this test, one dynamic sweep with the distinction loading system is applied to the aggregates of the ballast layer through the sleeper and rail fastening system and the effect of dynamic cyclic distribution is studied on the longitudinal resistance of ballast. The data logger gathers the information automatically through the computer connection and it processes the data related to the movement and applied forces and shows as output units. To record the required data during the test, two load cells and 6 LVDTs are needed. Therefore, in addition to longitudinal resistance, this apparatus can be used to determine the other results, such as track longitudinal stiffness and longitudinal displacement.Figure 22
Loading frame system [40].Figure 23
Track longitudinal resistance [40].(4) Track Lateral Resistance Test. For measuring the lateral resistance of railway track in the laboratory scale, there are three test techniques, including lateral track panel loading test (LTPT), single tie push test (STPT), and pendulum loading test apparatus, which are shown in Figures 24–26, respectively. Among them, STPT is the popular standard test to examine the single sleeper lateral resistance. This apparatus contains 3 principal sections, such as a transducer for calculating displacements, a loading Jack for imposing lateral forces, and a processor for collecting displacements and force amounts. The railway fastening systems of the middle sleeper were set free, and the hydraulic Jack and LVDTs were placed on the released sleeper. The Jack pulls the sleeper back from its support by enforcing the rail web. On another side of the sleeper, the displacement gauge stand is placed on the welded beam. This gauge calculates displacements while the sleeper moves. But on the contrary of STPT, LTPT apparatus operates the lateral loading on the railway track with three or five sleepers to evaluate the lateral resistance of the sleepers group, which is the advantage of this apparatus in proportion to STPT. However, the performance and operation of LTPT are similar to STPT, and both of them act statically. A load-cell is placed between the hydraulic cylinder and the rail to calculate the forces. On another side, the sleeper displacements are measured by separate LVDTs.Figure 24
The LTPT setup [41]: (a) hydraulic location. (b) LVDT position. (c) LVDT connection to sleepers.Figure 25
The STPT setup [42].Figure 26
Pendulum loading test apparatus [43].The lateral resistance of sleepers is the key parameter that guarantees the railway track lateral resistance and maintenance of geometric. The disadvantage of LTPT and STPT is that they only evaluate the lateral resistance statically and the railway track dynamic behavior is not investigated. To solve this problem, the pendulum loading test apparatus is suggested. This apparatus investigates the railway track lateral resistance dynamically. The other point of this apparatus is to investigate the interaction between the sleeper and the ballast regardless of rail.


## 4.2.2. Small Scale

Unlike previous models with a scale of 1 : 1, this section presents a railway track with smaller scales of 1 : 3 to 1 : 8.5. First, a single-sleeper track is described in which vertical and lateral loads are applied to a 1 : 3 scaled railway track with one sleeper. The weakness of this apparatus is the lack of longitudinal load simulation, which has a significant role in track maintenance. Then a 1 : 3 scaled railway track with 3 sleepers is presented, which longitudinal loading is not provided again in this section, and finally, a 1 : 5 scaled railway track is presented which the lateral resistance of railway track is measured under the influence of horizontal, lateral and vertical loads and the effects of vibration on the ballast can even be evaluated.(1) Single Sleeper Track. The single sleeper track test is shown in Figure 27. As illustrated, this apparatus includes the whole railway track with a single sleeper which is built with a scale of 1 : 3. The significant point is that with this apparatus, the characteristics of various types of sleeper and rail and different types of ballast material can be measured affected by static loading. The test bay comprises 2 plates in vertical sides with 500 cm length and 65 cm height, built with heavy steel parts and panels. These are placed at a 65 cm distance apart, related to a usual sleeper space, by steel ties at the base and other different locations. The test bay is intended to maintain situations as near as executable to plane strain. Wooden panels are placed on inside walls of the test bay and a double plastic sheeting layer was located between the sidewalls and the ballast in an effort for minimizing the friction of side interface. A soft board of a total thickness 2 cm is located at the bay bottom to illustrate a somewhat compressible subgrade and establish a frictional contact. The ballast is embedded in a minimum depth of 30 cm. A loading beam across the railheads transferred loads from the hydraulic actuator to the rail. Moment loadings would be exerted by changing the vertical hydraulic actuator eccentricity.Figure 27
(a) Schematic view, and (b) image of laboratory sleeper test [25].
(a)(b)(2) Ballast Railway Track with Three Sleeper. This apparatus is a reduced scale test with 3 sleepers to examine the dynamic behaviour and the ballasted track displacement, as shown in Figure 28. This test presents results such as settlements, pressures, accelerations and displacements that permit us to further understand the dynamic behaviour of a portion of ballasted railway tracks at a reduced-scale and to examine the displacement against the number of loading cycles. Also, it can determine depth settlement, cumulative deformation, and permanent axial deformation. In addition, the effect of wetting-drying cycles can be considered. The sleepers are loaded using 3 hydraulic jacks which present signals that have an M shape.Figure 28
Ballast railway track with three sleeper test [25].The test allows examining the ballasted railway track response because of the passage of high speed train. From bottom to top, the portion contains a layer of 60 cm height soil compacted in 3 layers with 20 cm height, a layer of compacted microballast (10 cm equivalent to 30 cm in actual scales) on which 3 sleepers are located based on the positions of hydraulic jacks, steel-created wedges placed on each sleeper for transferring loadings exerted by the jacks into the blocks and a layer of uncompacted microballast that is provided up to the blocks’ upper surface (8 cm). The distance of 2 sleepers is 20 cm at a reduced-scale, related to the actual-scale distance 60 cm. It is supported by a solid mass which comprises from bottom to top, a sand layer (250 cm), a reinforced concrete layer (100 cm) and a horizontal steel plate with 7 cm thick. The solid mass contained various substances is regarded as an infinite half-space [44]. The ballast layers are confined by inclined planes on 2 opposite sides which illustrate free edges. The other sides are confined by vertical steel plates (5 cm) that represent the end and the beginning of the portion. The hydraulic jacks are placed on a steel frame that is bolted on a horizontal steel plate.(3) Scaled Railway Track Models. As previously mentioned, the railway track with the laboratory scale provides the comprehensive investigation feasibility of it affected by the vertical, horizontal, and lateral displacement and the significant point is that the problems caused by field tests are also minimized. The smaller these tracks are made, the lower the construction and maintenance costs will be. After reviewing various researches, it was found that models with a scale of 1 : 5 and 1 : 8.5 have been made, which are smaller than previous scaled tracks and despite being small, it can be investigated the large area of ballast track and more sleepers. Figures 29 and 30 show the railway tracks with a scale of 1 : 5 in a rigid frame and without a frame, respectively. In model 1, the researcher investigates the lateral, vertical, and horizontal displacement affected by the horizontal loading. Also, it can be evaluated the effect of sleepers spacing, the effect of various types of sleepers (wooden, concrete, steel), and different types of ballast materials, and simulated the laboratory condition and freeze-dry cycles of winter seasons. But in model 2, it can be measured the sound radiation sound pressure level affected by vertical and lateral displacement. In general, the nature of both apparatuses is the same and the only difference is the rigid frame of model 1 that provides a platform for loading.Figure 29
1 : 5 scaled railway track in the rigid frame with five sleepers embedded in road beds [45].
(a)(b)(c)Figure 30
1 : 5 scaled railway track without the rigid frame with several sleepers [46].But there is a smaller model according to Figure31, which is sorted in both categories of scaled railway track and roller rig. In this model, it can be measured rolling noise and rail vibration, frequency spectra, and weighted sound pressure level. In addition, the effect of various fastening and sleepers, the sleepers’ space, and interaction between the track and locomotive can be achieved.Figure 31
The test rig for scale-model test with scale ratio of 1 : 8.5. (a) Overall view. (b) Measurement section [47].
(a)(b)


## 4.3. Roller Rig

One of the first studies to evaluate the flow performance of locomotives using a roller rig can be referred to the United Kingdom in 1904, which were continued with expanding of the railway system and related technologies in other countries of Asia, Europe, and American. This apparatus can be used in various forms and scales to investigate the railway vehicle dynamic and wheel-rail contact studies, which are described as follow. Also, the design of the roller rig is different according to the laboratory purposes. Among the parameters affecting the design of roller rigs, scale, test specimen and design concept can be mentioned [48, 49].The scale divides roller rig in 2 major classes: scaled and full-scale. Scaled designs present various advantages namely the easier implementation of parameter changes, lower space requirement, lower energy consumption, lower operation cost and lower investments. Nevertheless, just scaled models could be experimented, instead of whole vehicles or standard components. Also, interpreting the outcomes of the scaled test is not straight and is dependent on the selected scaling strategy [50].Based on the experimented specimens, the full vehicle test rigs, bogie, wheelset and single wheel are differentiated. The single wheelset setup remarkably enhances the system complication over the single wheel designs, however, it presents various advantages, particularly the capability to examine the interactions related to the suspension setup and dynamics of a single axle. Testing an assembled a whole vehicle or bogie notably enhances the complexity of the system over both the wheelset and single-wheel design, however, it potentially presents various additional benefits since vehicle rigs and bogie can be utilized to study the railway vehicle as a complete system.Many roller rig design concepts according to Figure32 are known that the most usual of these concepts is the vertical plane roller concept (Figure 32(a)). Other concepts including perpendicular roller (Figure 32(b)), internal roller (Figure 32(c)), or oscillating rail (Figure 32(d)) are commonly provided for special goal and the use of such designs is entirely uncommon in comparison with the standard vertical plane roller concept [51].Figure 32
Roller rig design concepts. (a) Vertical plane roller. (b) Perpendicular roller. (c) Internal roller. (d) Oscillating rail [51].
(a)(b)(c)(d)


### 4.3.1. Roller Rig for Railway Vehicle Dynamic Studies

Since single wheel rigs cannot be utilized to hunt researches, a single wheelset rig is minimum rig configuration in order to study a railway vehicle dynamic. However, the most usual rigs in this field are the full scale vertical roller rig for test the whole vehicles. The first apparatus of this type was utilized in the start of the twentieth century. Within those first experiments, the increasing intensity of lateral vibration when enhancing the peripheral speed of roller, known as hunting motion later, was identified. This event was regarded so considerable based on enhancing operation speed on the track and using roller rigs for railway vehicle running dynamic examination have started. The significance of these rigs has raised with high-speed rail developments. The most advanced rigs of this type, as are shown in Figure33, permit to be setup for various vehicle dimensions, loadings and wheel set gauge, and simulate vehicle running up to speed 500 km/h for broad ranges of track situations, including curved track and track irregularities. Since full bogie or vehicle can experiment under conditions same as a real track, experiments on this type of roller rigs came closer to reality. However, the construction and operating costs of these apparatuses are so great. Today, in the field of vehicle developments, the role of full-scale roller rigs is being exchanged by much more inexpensive computer-based simulations. Nonetheless, this kind of roller rigs still has a significant role in countries that begun to extend their own fleet of high speed vehicles [51].Figure 33
Two samples of full scaled roller dig in the world [48].Another test rig type that is used to study railway vehicle dynamics are scaled roller rigs, commonly designed for 1/5 to 1/3 scaled models of two axle bogies. Among significant disadvantages of a scaled system is the impossibility to test standard components directly and troubles with the interpretations of scaled-test outcomes in a full scale world. Using the scaled roller rigs in the development procedure of a specific vehicle is therefore considerably limited. Scaled roller rigs are generally constructed in research institutions or universities for developments and the test of entirely novel running gear concepts, namely, active-controlled drives of independently rotating wheels (Figure34(a)) [52], active-controlled wheel set steering (Figure 34(b)) [53, 54] or inverse thread wheel sets [51]. Moreover, a particular class of scaled experiment is testing on scaled tracks. In case of scaling of 1/5 or further, the whole track containing a straight and one or two curved parts can be located in a lab (Figure 35).Figure 34
(a) Roller rig with scale 1/5. (b) Roller rig with scale 1/3.5 [18].
(a)(b)Figure 35
(a) Steering railway bogies with scale 1/10 [55]. (b) Active steering vehicle with scale 1/5 [56].
(a)(b)


### 4.3.2. Roller Rigs for Wheel-Rail Contact Researches

Study of phenomena of wheel-rail contacts needs an accurate position of a roller and wheel, exact control of wheel and roller speed and accurate control of wheel loadings. The complex designs of a bogie enhance the complication to meet the needs. So roller rigs for examination the contact of wheel and rail are commonly in a single wheel form on roller or wheel set on rollers. Both full scale and scaled rigs are utilized (Figure36). Moreover, study of phenomena of wheel-rail contacts needs simulations of contact point factors as close as practicable to the track situations. The finite diameter of roller results in unavoidable faults of wheel-rail and wheel-roller contacts. In addition to standard vertical roller rigs, horizontal roller or inner roller concepts are also utilized (Figure 37(a)) since such designs permit errors decrease by enhancing the diameters of roller and keeping the overall rig diameters specified limits at the same time. A particular kind of test rig type is oscillating rail rig (Figure 37(b)).Figure 36
(a) Roller rig with small scale [57], (b) full-scaled roller rig (b) [58].
(a)(b)Figure 37
(a) Wheel-on-roller test rig with scale 1/5 [59], (b) full scale of wheel-on-roller test rig [60].
(a)(b)


## 4.3.1. Roller Rig for Railway Vehicle Dynamic Studies

Since single wheel rigs cannot be utilized to hunt researches, a single wheelset rig is minimum rig configuration in order to study a railway vehicle dynamic. However, the most usual rigs in this field are the full scale vertical roller rig for test the whole vehicles. The first apparatus of this type was utilized in the start of the twentieth century. Within those first experiments, the increasing intensity of lateral vibration when enhancing the peripheral speed of roller, known as hunting motion later, was identified. This event was regarded so considerable based on enhancing operation speed on the track and using roller rigs for railway vehicle running dynamic examination have started. The significance of these rigs has raised with high-speed rail developments. The most advanced rigs of this type, as are shown in Figure33, permit to be setup for various vehicle dimensions, loadings and wheel set gauge, and simulate vehicle running up to speed 500 km/h for broad ranges of track situations, including curved track and track irregularities. Since full bogie or vehicle can experiment under conditions same as a real track, experiments on this type of roller rigs came closer to reality. However, the construction and operating costs of these apparatuses are so great. Today, in the field of vehicle developments, the role of full-scale roller rigs is being exchanged by much more inexpensive computer-based simulations. Nonetheless, this kind of roller rigs still has a significant role in countries that begun to extend their own fleet of high speed vehicles [51].Figure 33
Two samples of full scaled roller dig in the world [48].Another test rig type that is used to study railway vehicle dynamics are scaled roller rigs, commonly designed for 1/5 to 1/3 scaled models of two axle bogies. Among significant disadvantages of a scaled system is the impossibility to test standard components directly and troubles with the interpretations of scaled-test outcomes in a full scale world. Using the scaled roller rigs in the development procedure of a specific vehicle is therefore considerably limited. Scaled roller rigs are generally constructed in research institutions or universities for developments and the test of entirely novel running gear concepts, namely, active-controlled drives of independently rotating wheels (Figure34(a)) [52], active-controlled wheel set steering (Figure 34(b)) [53, 54] or inverse thread wheel sets [51]. Moreover, a particular class of scaled experiment is testing on scaled tracks. In case of scaling of 1/5 or further, the whole track containing a straight and one or two curved parts can be located in a lab (Figure 35).Figure 34
(a) Roller rig with scale 1/5. (b) Roller rig with scale 1/3.5 [18].
(a)(b)Figure 35
(a) Steering railway bogies with scale 1/10 [55]. (b) Active steering vehicle with scale 1/5 [56].
(a)(b)


## 4.3.2. Roller Rigs for Wheel-Rail Contact Researches

Study of phenomena of wheel-rail contacts needs an accurate position of a roller and wheel, exact control of wheel and roller speed and accurate control of wheel loadings. The complex designs of a bogie enhance the complication to meet the needs. So roller rigs for examination the contact of wheel and rail are commonly in a single wheel form on roller or wheel set on rollers. Both full scale and scaled rigs are utilized (Figure36). Moreover, study of phenomena of wheel-rail contacts needs simulations of contact point factors as close as practicable to the track situations. The finite diameter of roller results in unavoidable faults of wheel-rail and wheel-roller contacts. In addition to standard vertical roller rigs, horizontal roller or inner roller concepts are also utilized (Figure 37(a)) since such designs permit errors decrease by enhancing the diameters of roller and keeping the overall rig diameters specified limits at the same time. A particular kind of test rig type is oscillating rail rig (Figure 37(b)).Figure 36
(a) Roller rig with small scale [57], (b) full-scaled roller rig (b) [58].
(a)(b)Figure 37
(a) Wheel-on-roller test rig with scale 1/5 [59], (b) full scale of wheel-on-roller test rig [60].
(a)(b)


## 5. Comparison and Better Decision-Making with the TOPSIS Model

Table4 shows a comparison of different laboratory apparatuses. Examination of all studies conducted on railway track loading tests resulted in 16 models of laboratory apparatuses that were compared according to the available information related to the apparatuses, in accordance with 7 criteria. These 7 criteria include the scale of the apparatuses, the range and system of the apparatuses (ballast, sleeper, rail and vehicle), loading direction (vertical, lateral and longitudinal), loading type (static or dynamic), simulation of environmental conditions (wet-dry (freeze-thaw) cycles and temperature degree), effects of geometric design and the possibility of vibration and noise. In general, it is an ideal apparatus that with small dimensions can perform dynamic and static loadings in all directions and can add the effects of environmental conditions and geometric design to the test conditions.Table 4
The comparison of various scaled railway laboratory tests.
Types of testsScaleRailway systemLoading directionLoading typeSimulation of environmental conditionsEffect of geometryVibration and noiseBallastSleepersRailsVehiclesVerticalLateralLongitudinalStaticDynamicWet-dry cyclesDegree conditionSmall scale triaxial1 : 05✓×××✓×××✓✓✓××Sleeper and ballasted tracks1 : 03✓✓××✓××✓×✓✓✓×Moving-wheel loading1 : 05✓✓××✓××✓✓✓××✓Ballast box1 : 01✓×××✓✓×✓×✓✓××Direct shear test machine1 : 01✓×××✓✓✓✓×××××Stone blowing1 : 01✓×××✓××✓×✓✓××Plate load tests (PLT)1 : 01✓×××✓××✓×✓✓××Fatigue test1 : 01✓✓✓×✓××✓✓××××Railway test facility1 : 01✓✓✓×✓××✓×××××Railway track panel with longitudinal and LTPT test1 : 01✓✓✓××✓✓✓✓✓✓✓×Single sleeper track1 : 03✓✓✓×✓✓×✓×××××Ballasted railway track with three sleepers1 : 03✓✓✓×✓×××✓✓✓✓×Scaled railway track models1 : 05✓✓✓××✓✓×✓✓✓✓✓Roller rig1 : 05××✓✓✓✓××✓✓✓×✓Active steering vehicle1 : 05××✓✓✓✓××✓××✓×Wheel-on-roller test rig1 : 05××✓✓✓✓×✓✓✓✓×✓As can be seen in Table4, each apparatus has only a part of the stated specifications, in other words, the apparatuses are complementary to each other. For example, in laboratory apparatuses related to ballast tests, only the ballast layer is examined, and in most of them, the focus is only on vertical loading, or only a limited number of apparatuses, including moving wheel loading, scaled railway track model, roller rig and wheel on roller test rig have the ability to measure vibration and noise due to loading, which will make this criterion important. Among the apparatuses that perform static and dynamic tests on the track panel, the two systems of scaled railway track model and railway track panel with longitudinal and LTPT test had more capabilities than the others.In order to better compare and also make a more appropriate decision to select the most efficient apparatuses, they were scored according to their capabilities according to the 7 criteria examined in Table4 and then analyzed using the TOPSIS model. This technique was proposed in 1981 by Hwang and Yoon. In this technique, m options are examined by n indices and each problem can be regarded as a geometric system comprising m points in a n-dimensional space. This technique is according to the notion that the selected option should have the shortest distance to the positive ideal solutions (best possible case, Aj+) and the greatest distance to the negative ideal solutions (worst possible case, Aj−). It is supposed that the desirability of each index is decreasing or increasing uniformly [61]. Solving a problem by TOPSIS analysis consists of 6 steps. In the first step, the matrix D is transformed into an unscaled matrix using Euclidean norm:(6)rij=rij∑i=1mrij21/2,i=1,…,m.The resulting matrix is calledND. Then a weighted unscaled matrix is achieved.(7)V=ND×Wn×n,Where V is a weighted unscaled matrix and W indicates a diagonal matrix of weights achieved for the indices. In the third stage, the positive and negative ideal solutions (best and worst possible case, respectively) are determined [62, 63]:(8)A+=maxivij|j∈J1,minvij|j∈J2|i=1,2,…,m,A−=minivij|j∈J1,maxvij|j∈J2|i=1,2,…,m,in which J1=1.2…n|associated with the criteriahaving a positive impact and J2=1.2…n|associated with the criteria having a negative impact.In the fourth step, the distance size according to the Euclidean norm for the negative ideal solution and the positive option and the same size for the positive ideal solution and the negative option is obtained as follows [64]:(9)di+=∑i=1nvij−vj+21/2,i=1,2,…,m,di−=∑i=1nvij−vj−21/2,i=1,2,…,m.In the fifth step, the relative proximity ofAj to the ideal solution is calculated as equation (9) [65]:(10)Ci=di−di−+di+,i=1,2,…,m.IfAi=Ai+, then di+=0 and Ci=1, and if Ai=Ai−, then di−=0 and Ci=0, so each Aj option is closer to the ideal solution, its Ci value will be closer to one. Finally, in the last step, the options are ranked, and the existing options can be ranked based on the descending order of Ci. Tables 5–8 show the results of the normal matrix, weight matrix, optimal solution and distance size for each of the 16 test apparatuses according to the 7 criteria. Also, the results of weight values of 7 criteria affecting the decision making and final rankings of laboratory apparatuses are shown according to Figures 38 and 39.Table 5
Normal matrix results.
Test typeScaleRailway systemLoading directionLoading typeSimulation of environmental conditionsEffect of geometric designVibration and noiseSmall scale triaxial0.35110.11250.14910.24250.312300Sleeper and ballasted tracks0.23410.2250.14910.12130.31230.44720Moving-wheel loading0.35110.2250.14910.36380.156200.5Ballast box0.1170.11250.29810.12130.312300Direct shear test machine0.1170.11250.44720.1213000Stone blowing0.1170.11250.14910.12130.312300Plate load tests (PLT)0.1170.11250.14910.12130.312300Fatigue test0.1170.33750.14910.3638000Railway test facility0.1170.33750.14910.1213000Railway track panel (longitudinal & LTPT)0.1170.33750.29810.36380.31230.44720Single sleeper track0.23410.33750.29810.1213000Ballasted railway track with three sleepers0.23410.33750.14910.24250.31230.44720Scaled railway track models0.35110.33750.29810.36380.31230.44720.5Roller rig0.35110.2250.29810.24250.312300.5Active steering vehicle0.35110.2250.29810.242500.44720Wheel-on-roller test rig0.35110.2250.29810.36380.312300.5Table 6
Weighted matrix results.
Test typeScaleRailway systemLoading directionLoading typeSimulation of environmental conditionsEffect of geometric designVibration and noiseSmall scale triaxial0.01170.00290.00330.00770.036600Sleeper and ballasted tracks0.00780.00580.00330.00390.03660.1570Moving-wheel loading0.01170.00580.00330.01160.018300.2092Ballast box0.00390.00290.00660.00390.036600Direct shear test machine0.00390.00290.00990.0039000Stone blowing0.00390.00290.00330.00390.036600Plate load tests (PLT)0.00390.00290.00330.00390.036600Fatigue test0.00390.00870.00330.0116000Railway test facility0.00390.00870.00330.0039000Railway track panel (longitudinal and LTPT)0.00390.00870.00660.01160.03660.1570Single sleeper track0.00780.00870.00660.0039000Ballasted railway track with three sleepers0.00780.00870.00330.00770.03660.1570Scaled railway track models0.01170.00870.00660.01160.03660.1570.2092Roller rig0.01170.00580.00660.00770.036600.2092Active steering vehicle0.01170.00580.00660.007700.1570Wheel-on-roller test rig0.01170.00580.00660.01160.036600.2092Table 7
Optimal solution results.
Optimal solutionScaleRailway systemLoading directionLoading typeSimulation of environmental conditionsEffect of geometric designVibration and noisePositive0.01170.00870.00990.01160.03660.1570.2092Negative0.00390.00290.00330.0039000Table 8
Distance size results.
Test typeDistance sizePositiveNegativeSmall scale triaxial0.01170.0029Sleeper and ballasted tracks0.00780.0058Moving-wheel loading0.01170.0058Ballast box0.00390.0029Direct shear test machine0.00390.0029Stone blowing0.00390.0029Plate load tests (PLT)0.00390.0029Fatigue test0.00390.0087Railway test facility0.00390.0087Railway track panel (longitudinal and LTPT)0.00390.0087Single sleeper track0.00780.0087Ballasted railway track with three sleepers0.00780.0087Scaled railway track models0.01170.0087Roller rig0.01170.0058Active steering vehicle0.01170.0058Wheel-on-roller test rig0.01170.0058Figure 38
Weight values of 7 criteria affecting the selection of railway track laboratory apparatuses.Figure 39
Final ranking of laboratory apparatuses.The results of TOPSIS analysis of 7 criteria affecting the selection of laboratory-scale apparatus for simulation and static and dynamic loads showed that the three variables of environmental conditions, geometric designs and test to measure the vibration and noise parameters were far from the ideal negative solution and were positive options and thus have higher weight values than other variables. Therefore, according to the final weight values obtained, the laboratory apparatuses of scaled railway track model, wheel on roller test rig and roller rig had the first to third ranks, respectively.


## 6. Conclusion

This study focuses on different laboratory methods of railway tracks at different scales. First, by reviewing various studies, the forces applied on the railway, including longitudinal, vertical and lateral forces, were introduced, and then the lateral, longitudinal and creep resistances caused by the forces applied on the railway tracks were examined. So, it was indicated that(i)
There are various apparatuses to measure the parameters affecting the design of railway tracks, including railway settlement, railway and sleeper displacements, lateral and longitudinal resistances, vibration and noise due to different loadings, some of which are conducted in field conditions and the rest are performed in the laboratory space.(ii)
Due to the high range of apparatuses on a laboratory scale, the study of field apparatuses in this research was avoided. Of course, it should be noted that in field tests, the purpose is the measurement of the mentioned parameters under the influence of the train passing. But in the laboratory condition, in order to measure these parameters, the apparatuses are able to simulate the forces applied on the railway track.(iii)
Since each of these apparatuses follows a specific range, therefore, they are divided into different categories, the first of which examines the parameters affecting the ballast layer and most are capable of vertical loading. The weakness of these tests is the lack of examination of other systems involved in railway tracks, including sleepers and rails.(iv)
The second category of apparatuses includes tests that are mounted on a large-scale (1 : 1) or small-scale (1 : 3 to 1 : 8.5) railway panel, and simulate longitudinal, vertical, and lateral loads that vary in different scales and sizes of laboratory apparatuses.(v)
In the third category, the roller rigs can be mentioned that simulate the interaction between the rail and the wheel and the effect of the vehicle on the railway track. But the weakness that can be introduced is the lack of investigation of the impact of the vehicle on the sleepers and the underlying layers of the railway track.(vi)
In order to compare different laboratory apparatuses and make better decisions for selecting more efficient apparatuses for simulating the loading of railway tracks, 7 criteria were examined. By the use of TOPSIS analysis, apparatuses of scaled railway track model, wheel on roller test rig and roller rig ranked first to third, respectively.(vii)
Scaled railway track models are distinguished by the fact that in addition to having a small scale and therefore lower construction cost, they also have the ability to simulate lateral and longitudinal forces and can measure noise and vibration from different loadings on different layers of the railway track.(viii)
Due to the small scale and being in rigid frame, different geometric designs and environmental conditions such as wet and dry cycles, freeze and thaw cycles and different temperatures can be simulated and their effect on the railway track can be examined. Therefore, researchers in future studies can turn this test into a comprehensive laboratory apparatus by eliminating other weaknesses, including the possibility of simulating vertical and static forces. Moreover, other machine learning methods can be incorporated into the proposed approaches to obtain more accurate results [66, 67]. By understanding users' perceptions regarding the improvement of railways, these methods in conjunction with a survey analysis can also improve work zone safety [68].



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