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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 track
All 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 Figure 1. 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 omitted; refer to PDF]
As illustrated in Figure 10, 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 omitted; refer to PDF]
As can be seen in Figure 11, 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 omitted; refer to PDF]
(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 omitted; refer to PDF]
(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 omitted; refer to PDF]
(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 omitted; refer to PDF]
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.
[figures omitted; refer to PDF]
(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 omitted; refer to PDF]
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.
[figures omitted; refer to PDF]
[figure omitted; refer to PDF]
But there is a smaller model according to Figure 31, 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.
[figures omitted; refer to PDF]
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 Figure 32 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].
[figures omitted; refer to PDF]
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 Figure 33, 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 omitted; refer to PDF]
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 (Figure 34(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).
[figures omitted; refer to PDF]
[figures omitted; refer to PDF]
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 (Figure 36). 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)).
[figures omitted; refer to PDF]
[figures omitted; refer to PDF]
5. Comparison and Better Decision-Making with the TOPSIS Model
Table 4 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 tests | Scale | Railway system | Loading direction | Loading type | Simulation of environmental conditions | Effect of geometry | Vibration and noise | |||||||
| Ballast | Sleepers | Rails | Vehicles | Vertical | Lateral | Longitudinal | Static | Dynamic | Wet-dry cycles | Degree condition | ||||
| Small scale triaxial | 1 : 05 | ✓ | × | × | × | ✓ | × | × | × | ✓ | ✓ | ✓ | × | × |
| Sleeper and ballasted tracks | 1 : 03 | ✓ | ✓ | × | × | ✓ | × | × | ✓ | × | ✓ | ✓ | ✓ | × |
| Moving-wheel loading | 1 : 05 | ✓ | ✓ | × | × | ✓ | × | × | ✓ | ✓ | ✓ | × | × | ✓ |
| Ballast box | 1 : 01 | ✓ | × | × | × | ✓ | ✓ | × | ✓ | × | ✓ | ✓ | × | × |
| Direct shear test machine | 1 : 01 | ✓ | × | × | × | ✓ | ✓ | ✓ | ✓ | × | × | × | × | × |
| Stone blowing | 1 : 01 | ✓ | × | × | × | ✓ | × | × | ✓ | × | ✓ | ✓ | × | × |
| Plate load tests (PLT) | 1 : 01 | ✓ | × | × | × | ✓ | × | × | ✓ | × | ✓ | ✓ | × | × |
| Fatigue test | 1 : 01 | ✓ | ✓ | ✓ | × | ✓ | × | × | ✓ | ✓ | × | × | × | × |
| Railway test facility | 1 : 01 | ✓ | ✓ | ✓ | × | ✓ | × | × | ✓ | × | × | × | × | × |
| Railway track panel with longitudinal and LTPT test | 1 : 01 | ✓ | ✓ | ✓ | × | × | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | × |
| Single sleeper track | 1 : 03 | ✓ | ✓ | ✓ | × | ✓ | ✓ | × | ✓ | × | × | × | × | × |
| Ballasted railway track with three sleepers | 1 : 03 | ✓ | ✓ | ✓ | × | ✓ | × | × | × | ✓ | ✓ | ✓ | ✓ | × |
| Scaled railway track models | 1 : 05 | ✓ | ✓ | ✓ | × | × | ✓ | ✓ | × | ✓ | ✓ | ✓ | ✓ | ✓ |
| Roller rig | 1 : 05 | × | × | ✓ | ✓ | ✓ | ✓ | × | × | ✓ | ✓ | ✓ | × | ✓ |
| Active steering vehicle | 1 : 05 | × | × | ✓ | ✓ | ✓ | ✓ | × | × | ✓ | × | × | ✓ | × |
| Wheel-on-roller test rig | 1 : 05 | × | × | ✓ | ✓ | ✓ | ✓ | × | ✓ | ✓ | ✓ | ✓ | × | ✓ |
As can be seen in Table 4, 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 Table 4 and then analyzed using the TOPSIS model. This technique was proposed in 1981 by Hwang and Yoon. In this technique,
The resulting matrix is called
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]:
In the fifth step, the relative proximity of
If
Table 5
Normal matrix results.
| Test type | Scale | Railway system | Loading direction | Loading type | Simulation of environmental conditions | Effect of geometric design | Vibration and noise |
| Small scale triaxial | 0.3511 | 0.1125 | 0.1491 | 0.2425 | 0.3123 | 0 | 0 |
| Sleeper and ballasted tracks | 0.2341 | 0.225 | 0.1491 | 0.1213 | 0.3123 | 0.4472 | 0 |
| Moving-wheel loading | 0.3511 | 0.225 | 0.1491 | 0.3638 | 0.1562 | 0 | 0.5 |
| Ballast box | 0.117 | 0.1125 | 0.2981 | 0.1213 | 0.3123 | 0 | 0 |
| Direct shear test machine | 0.117 | 0.1125 | 0.4472 | 0.1213 | 0 | 0 | 0 |
| Stone blowing | 0.117 | 0.1125 | 0.1491 | 0.1213 | 0.3123 | 0 | 0 |
| Plate load tests (PLT) | 0.117 | 0.1125 | 0.1491 | 0.1213 | 0.3123 | 0 | 0 |
| Fatigue test | 0.117 | 0.3375 | 0.1491 | 0.3638 | 0 | 0 | 0 |
| Railway test facility | 0.117 | 0.3375 | 0.1491 | 0.1213 | 0 | 0 | 0 |
| Railway track panel (longitudinal & LTPT) | 0.117 | 0.3375 | 0.2981 | 0.3638 | 0.3123 | 0.4472 | 0 |
| Single sleeper track | 0.2341 | 0.3375 | 0.2981 | 0.1213 | 0 | 0 | 0 |
| Ballasted railway track with three sleepers | 0.2341 | 0.3375 | 0.1491 | 0.2425 | 0.3123 | 0.4472 | 0 |
| Scaled railway track models | 0.3511 | 0.3375 | 0.2981 | 0.3638 | 0.3123 | 0.4472 | 0.5 |
| Roller rig | 0.3511 | 0.225 | 0.2981 | 0.2425 | 0.3123 | 0 | 0.5 |
| Active steering vehicle | 0.3511 | 0.225 | 0.2981 | 0.2425 | 0 | 0.4472 | 0 |
| Wheel-on-roller test rig | 0.3511 | 0.225 | 0.2981 | 0.3638 | 0.3123 | 0 | 0.5 |
Table 6
Weighted matrix results.
| Test type | Scale | Railway system | Loading direction | Loading type | Simulation of environmental conditions | Effect of geometric design | Vibration and noise |
| Small scale triaxial | 0.0117 | 0.0029 | 0.0033 | 0.0077 | 0.0366 | 0 | 0 |
| Sleeper and ballasted tracks | 0.0078 | 0.0058 | 0.0033 | 0.0039 | 0.0366 | 0.157 | 0 |
| Moving-wheel loading | 0.0117 | 0.0058 | 0.0033 | 0.0116 | 0.0183 | 0 | 0.2092 |
| Ballast box | 0.0039 | 0.0029 | 0.0066 | 0.0039 | 0.0366 | 0 | 0 |
| Direct shear test machine | 0.0039 | 0.0029 | 0.0099 | 0.0039 | 0 | 0 | 0 |
| Stone blowing | 0.0039 | 0.0029 | 0.0033 | 0.0039 | 0.0366 | 0 | 0 |
| Plate load tests (PLT) | 0.0039 | 0.0029 | 0.0033 | 0.0039 | 0.0366 | 0 | 0 |
| Fatigue test | 0.0039 | 0.0087 | 0.0033 | 0.0116 | 0 | 0 | 0 |
| Railway test facility | 0.0039 | 0.0087 | 0.0033 | 0.0039 | 0 | 0 | 0 |
| Railway track panel (longitudinal and LTPT) | 0.0039 | 0.0087 | 0.0066 | 0.0116 | 0.0366 | 0.157 | 0 |
| Single sleeper track | 0.0078 | 0.0087 | 0.0066 | 0.0039 | 0 | 0 | 0 |
| Ballasted railway track with three sleepers | 0.0078 | 0.0087 | 0.0033 | 0.0077 | 0.0366 | 0.157 | 0 |
| Scaled railway track models | 0.0117 | 0.0087 | 0.0066 | 0.0116 | 0.0366 | 0.157 | 0.2092 |
| Roller rig | 0.0117 | 0.0058 | 0.0066 | 0.0077 | 0.0366 | 0 | 0.2092 |
| Active steering vehicle | 0.0117 | 0.0058 | 0.0066 | 0.0077 | 0 | 0.157 | 0 |
| Wheel-on-roller test rig | 0.0117 | 0.0058 | 0.0066 | 0.0116 | 0.0366 | 0 | 0.2092 |
Table 7
Optimal solution results.
| Optimal solution | Scale | Railway system | Loading direction | Loading type | Simulation of environmental conditions | Effect of geometric design | Vibration and noise |
| Positive | 0.0117 | 0.0087 | 0.0099 | 0.0116 | 0.0366 | 0.157 | 0.2092 |
| Negative | 0.0039 | 0.0029 | 0.0033 | 0.0039 | 0 | 0 | 0 |
Table 8
Distance size results.
| Test type | Distance size | |
| Positive | Negative | |
| Small scale triaxial | 0.0117 | 0.0029 |
| Sleeper and ballasted tracks | 0.0078 | 0.0058 |
| Moving-wheel loading | 0.0117 | 0.0058 |
| Ballast box | 0.0039 | 0.0029 |
| Direct shear test machine | 0.0039 | 0.0029 |
| Stone blowing | 0.0039 | 0.0029 |
| Plate load tests (PLT) | 0.0039 | 0.0029 |
| Fatigue test | 0.0039 | 0.0087 |
| Railway test facility | 0.0039 | 0.0087 |
| Railway track panel (longitudinal and LTPT) | 0.0039 | 0.0087 |
| Single sleeper track | 0.0078 | 0.0087 |
| Ballasted railway track with three sleepers | 0.0078 | 0.0087 |
| Scaled railway track models | 0.0117 | 0.0087 |
| Roller rig | 0.0117 | 0.0058 |
| Active steering vehicle | 0.0117 | 0.0058 |
| Wheel-on-roller test rig | 0.0117 | 0.0058 |
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].
Disclosure
In this study, Iranian governmental organizations have not been partners and sponsors, and this study is purely studious.
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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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Details
; Habibzadeh, Mohammad 1
; Seyed Mohsen Hosseinian 1
; Salehfard, Reza 1 1 School of Civil Engineering, Iran University of Science and Technology (IUST), Tehran, Iran