The Long-Term Inspection and Monitoring of Transition Zones with a Sudden Change in Railway Track Stiffness

Abstract

Transition zones are located at points on a track where there has been a change in the main composition of the railway infrastructure; as such, there are many sections that undergo a sudden change in the stiffness of the structures built. When trains are running, a longitudinal shockwave is created by the wheels, hitting these building objects with a greater stiffness and deforming the surroundings of these zones. The greatest amount of attention should be paid to the transition points from the fixed track to the classic track with a track bed, including objects of the railway substructure, such as bridges and portals of tunnels. As part of the research on the main corridor lines, long-term inspection and monitoring studies were carried out using a trolley with a continuous measurement system; height changes in the deflections of rails are evidence of their behaviour. The measurements took place on a fixed track and a track with ballast. The changes in the height jumps between the fixed railway track and the track with a gravel bed are significant. These height deflections allow designers to develop new, more durable construction designs.

1. Introduction

Increasing the quality of the design and construction of the tracks for railway vehicles and improving the safety and comfort of their passengers during operation are among the main tasks of the international railway group (UIC) and the European Community. Depending on the quality of the designed track, the smooth operation of train sets may be disturbed by some objects that are built into the tracks. Shockwaves can hit these objects, spreading from the wheels. These objects are called transition zones (TZs), in which the rail’s upper structure and lower substructure objects suddenly change the track’s stiffness. They are located on all railway lines as a transition between a fixed railway track (FRT) and a classic one with a gravel bed (railway ballast), as well as between tracks built on gravel paths and reinforced concrete structures that are embedded into the railway substructure of the fixed tracks of bridges or tunnel portals.

We know from railway practice and research tasks that these locations generate complications in the structural design, construction, and operation of train sets over these short sections. If a train set travels on a fixed track and its section ends, it will hit the ground in the transition zone, i.e., the track’s ballast bed. This transition phenomenon also occurs in bridge structures. Trains must also travel in reverse along this transition section; as a result, impact forces are generated, which cause the track’s geometric directional and vertical sections to collapse. The same applies if objects (for example, bridges, tunnel portal foundations, tunnel tubes, etc.) are located on the railway substructure with a continuous track bed above it.

If the rail bed was continuous, two options would arise. If it was a fixed track, it would be in a long-term, high-quality, sustainable condition, but it could not be tamped. Similarly, if the track was on a continuous track bed, the directional and height deformations caused by traffic would affect the track’s position and height. Still, the track would be tamped regularly with mechanical tamping machines to the designed or required position and height. In both cases, it would be necessary to ensure that the time interval between the track’s position and height adjustments is as long as possible. This could be achieved using high-quality constructed track sections, with an emphasis on the behaviour of objects in transition zones. The malfunction of a part of a train set or the poor condition of the structures on the track may cause accidents in these sections. The primary cause—the “trigger”—may be hidden in these poor directional and elevation positions of the track in each section with deformations, i.e., they may trigger the cause of the accident. Conversely, in a poorly constructed transition zone, the forces acting on the railway track can degrade it and the rolling stock of trains.

Sanudo et al. [1] addressed transition zones with a more rigid railway body by adding additional rails in their research. The data were measured on the track using the Dinotrans system. This is one of the new solutions that can be applied at the transition from a solid slab to a rail bed, with flexible rail fastening. Accelerometers and extensometer sensors monitor these data.

Another option for modifying the transition zone involves the material composition of the underlying layers and the building objects in the lower part of the railway line, as solved in a study by Jain et al. [2]. This comprehensive study addressed the persistent issue of transition zone degradation in railways, evaluating the efficacy of the most commonly used mitigation measures and proposing a novel safe hull-inspired energy-limiting design for a transition zone structure. The authors of [3] solved the railway transition zone issue, wherein there was a greater degradation of objects in these stressed sections, and tried to delay these processes, leading to an uneven track geometry. Deformation resistance is important for maintaining the long-term, stable position and height of the tracks in this force-intensive section.

Deformed areas of the railway body, such as sections in front of and behind bridge structures, which are prone to differential settlement and heave during train loading, are addressed in a study by Akshay et al. [4]. Structural layers and embedded structures degrade faster in these critical parts, such as at the supports of railway bridges, due to the deterioration of their deformation resistance in the transition zones. New designs of these track sections at bridges are important for improving the stiffness of the track while preventing deformations.

The problem posed by high-speed railway transition zones was dealt with by Ping H. et al. [5] within the framework of sections near bridges and culverts. They monitored the dynamic response, structural parameters, and material properties of individual layers when changing their proposed parameters to improve the stiffness of railway infrastructure.

Park S. et al. [6] solved a problem regarding the Rheda 2000-type rigid track in the transition zone areas on high-speed lines at speeds of 280–300 km/h. The numerical analysis included a definition of the rigid reinforced concrete track and the moving load of the railway line. The developed model demonstrated values that are determined in the transition section of the railway line. These transition zones were vulnerable, and the result was the deformation of the track geometry.

Fortunato E. et al. [7] addressed transition zones near bridges and other construction objects of the lower railway substructure in their study. With an increased stress on railway structures from moving loads, the degradation of these structures occurs. These objects in transition zones need to be regularly monitored, maintained, or reconstructed. Therefore, the authors addressed the task of reducing the financial costs for maintaining these objects by employing model design. Wang W. et al. [8] monitored transition zones in their research, not only from the point of view of the structural layers but also from the perspective of the possible increased humidity in these structures. A high humidity also presents significant problems for the transition zones, which are the causes of the degradation of building elements in the track; increased humidity causes a decrease in the stiffness of the structural layers.

Lee J. et al. [9] delt with the solution of structural parts to ensure a gradual change in the stiffness of the railway. They inserted various elements into the transition zone to increase the stiffness of the structural layers. Experiments were carried out on the track at the transition from solid slab to gravel, both at bridges and in tunnels.

Tan S.H. et al. [10] presented embedded reinforced concrete slabs that were highly effective in reducing the vibration height response in transition zones. The authors pointed out that, if the resistance of these building structures was improved, then the vibration resistance would also be better under moving loads during the movement of train sets.

It is currently impossible to add references from abroad, as none exist on the given topic, i.e., addressing the geometry of the track and the safe passage of train sets. These references always solve only one problem on a tiny sample (sometimes the transition zone only), but from a completely different perspective from the researcher. The geometric position and track height of both R/L rails (i.e., track gradients) are also important, as heavy passenger and freight trains run at different high speeds. It would be best if all authors addressed everything at once and did not just one select topic in the transition zones. This paper describes the results of our research in determining the behaviour of track geometry in operation from a long-term perspective.

2. Research Design and Methods

Monitoring the health condition of the objects of the railway’s lower substructure of the transition zones would be appropriate; however, the monitoring is carried out on the railways as follows. The monitoring and evaluation of the conditions of the track of the entire carriageway takes place with the help of hand-held measuring devices (trolleys), such as KRAB [11] and measuring waggons, based on the measurements of the track parameters. The reconstruction measures of the lower substructure objects will only be started based on the detected deformations in the geometrical position of the track. The limits for corrective interventions in the track are given in the prescribed criteria of standards and regulations. In our research, we perform very detailed measurements of the geometric position and heights of the track on the transition zones.

If transition zones do not maintain the designed or prescribed geometrical position and heights of the tracks for a long time, this results in increased costs for their maintenance during their operation and unwanted deformations of parts of rolling stock driving on them from wheel impacts.

The procedures and accuracy of KRAB measurements have been verified, e.g., in [12,13]. During this research, numerical models of transition zones and measurements in the track were developed. The models confirmed the accuracy and correctness of the measurements. Another comparison with geodetic methods found that the KRAB system measured heights more accurately (±1 mm; mean error of height determination at the measured point) than those determined using geodetic measurements. This is a relative determination of heights but is performed on a long 200–600 m section of the track axis on both R/L rails (right/left). For example, if we look at the figures in this paper, the individual measurements follow each other at various time intervals. This means that each new measurement confirmed the values of the previous measurement.

Research has been carried out at the Department of Railway Engineering for several years, since 2013, in relation to the transition zones near the railway tunnel in the town of Trencianske Bohuslavice, as well as at the railway bridge over the Vah River in Trencin, both with an FRT with a speed of 160 km/h, as shown in Figure 1a. The research involves continuous measurements, ensuring a comprehensive understanding of the transition zones. Currently, a new experimental section near the Milochov bridge near the city of Puchov is being prepared with a continuous gravel bed. The measurements were carried out continuously using the KRAB measuring device [11]. In addition, sensors for measuring the forces between the rail and sleeper, as well as between acceleration and speeds, will be built into the upper part of the railway structure (for measurements due to take place from 2025). Measurements will be made while the train sets are passing. In the first phase at the Milochov bridge (in the 3rd-stage measurement), based on previous monitoring scenarios, the positions of these devices (sensors) in the transition zones will be determined; preparation is now underway.

In the inspection and monitoring courses, research is conducted on the transition zones that are built into the railway’s upper and lower structures. The train creates a longitudinal shock wave, which hits a solid object, such as a reinforced concrete slab (FRT) or, for example, the construction of railway bridges, tunnel portals, etc. The research was carried out on the track with these objects, either with FRT or a classic track bed.

Experimental measurements were performed continuously in the measurement and evaluation step (at a length of 25 cm), as shown in Figure 1b. The accuracy of the KRAB measuring device is reliable (±1 mm) in relation to the average height determination error due to the deflection of the LHD rails R/L, which is also documented by the graphic outputs in individual stages. The same waves are recorded everywhere and the curves of the graphs are transformed gradually (as was also confirmed in previous research measurements). Each new stage of measurement on the transition zone, as part of the monitoring, confirmed the curve of the last measurements and certain new developments of the LHD of the deformation of the heights of the rails over time.

$$z\left(x\right)=y\left(x\right)-[{\displaystyle \frac{b}{a+b}}.y\left(x-a\right)+{\displaystyle \frac{a}{a+b}}.y(x+b)]$$

(1)

Figure 1c shows the procedure for evaluating the measurements of the directional and elevation solutions of the track on a three-point asymmetric chord (a + b). The inserted function relationship (1), which is known as the transfer function, is a function of the given calculation of the z(x), which derives from the input signal y(x). In the case of measurements with the KRAB measuring trolley, this chord is 2.5 m long (a = 1.15 m; b = 3.1 m), and the entire device works sequentially with a step of 0.25 m over the whole length of the measured section. If we add another arm to the trolley, the chord will be 5.6 m (a = 2.5; b = 3.1). The actual transition from the track domain to the wave domain is performed by the Fourier transformation, resulting in the transfer function of the repulsion being within a three-point asymmetric chord (for both directional and height ratios of the track geometry). The formulas are the exclusive subject of the manufacturer’s trade secret [11], and the resulting numerical outputs from the measurements are in the required form in the KRAB device.

The authors of [1,2,3,4,5,6,7,8,9,10] dealt with transition zones from specific perspectives because various forces were manifested in these parts of the tracks, which caused the degradation of individual structures. Significant findings included the determination of the current condition of the tracks on the railway line as a result of the action of these particular phenomena. The resulting deformations of the track geometry, its position, and its height were dealt with by the research activity of the Department of Railway Construction [12,13,14], which was carried out on transition zones with fixed tracks (FRT only). The research presents the transition of a solid reinforced concrete track to a gravel bed, including structures with FRT, such as small and large bridges, tunnel portals, etc., for speeds of 160 km/h. These places with a sudden change in track stiffness were monitored continuously over time, at various intervals, using the KRAB device at a specific measurement step.

Hodas S. et al. [12] monitored the deformations of the track axes in the transition zones, where the solid reinforced concrete slab began or ended, and the transition to the classic track with a gravel bed occurred. Monitoring was carried out using the KRAB device, and heights were determined using geodetic measurements. The resulting values of both methods were compared with each other.

Another undesirable phenomenon for transition zones is the disruption of structural layers by frost in the winter, which Hodas S. addressed in their research [13]. The paper describes the possibilities of the frost protection of structural layers using materials with low thermal conductivity coefficients. New frost protection solutions were proposed using numerical models.

Monitoring the transition zones of bridge structures was discussed by Hodas S. et al. [14], where they stated that the same phenomena occurred in every transition zone.

Basic experimental measurements, as part of the monitoring process, were carried out after the new corridor line was handed over to operation near the Milochov bridge, in both 4/2023 and 9/2023. Sense are expected to be installed in the upper rail superstructure to these sections to allow for further measurements in 2025 (these experimental sections were prepared during 2024 and 2025).

The various types of materials that can be built into the track in the transition zones are tested at the testing stands in the laboratories of the Department of Railway Construction.

Continuous monitoring by sensors in transition zones would be the ideal approach, but it would involve monitoring only a few objects (from 1 to 3 objects), which would incur high financial costs. However, our task is to monitor several hundred transition zones in different railway infrastructure lines based on the actual track geometry (position and height). These are sections between the solid reinforced concrete track and the so-called “jump” to the track bed. Conversely, train sets must “step up” from the gravel bed to the solid track; these situations are observed near bridges, tunnel portals, and culverts.

Analyses of in-depth research solutions are elaborated in detail in Section 4, discussing common characteristics of the behaviour of structural objects in transition zones. The resulting indicators have been created based on the long-term continuous measurements with the KRAB measuring trolley that have been carried out since 2013. The result, in addition to the objects’ behaviour at the interface between the fixed railway track and the track bed (the same applies to bridges, tunnels, culverts, and other objects in the lower structure of the railway bed) is a set of information, i.e., from which it is possible to determine critical locations for the placement of sensors for various physical measurements. Unfortunately, in practice, no one will place them in every transition zone (financially demanding) based on the monitoring results. Designers can find critical points of sudden changes in stiffness and can design higher-quality and more durable structures for transition zones. This will increase safety and comfort when passing through these transition zones in the railway infrastructure (not forgetting the high speeds of heavy passenger and freight trains).

3. Results

A new section with input measurements on the track, exclusively with a track bed, was also included (previous monitoring since 2013 has always been conducted with FRT). So far, two measures have been carried out on the new sections, i.e., after putting the railway line into operation and then after five months of operation. The characteristic features of the shape of the track geometry include the longitudinal height changes in the deflections (LHDs) of the R/L rails. In the transition zone and its adjacent sections, experimental measurements are carried out in the railway substructure at points from S1 to S4, i.e., determined in certain characteristic places observed by long-term monitoring in particular transition zones in 2013–2024 [13,14], according to KRAB measuring.

In 2020–2023, the individual tasks of these activities were covered by Izvolt L. et al. [15]. The measurement locations of sensors from S1 to S4 in Figure 2 were built into the lower layers of the railway structure from the beginning of the measurements but do not belong to the monitoring process that uses the KRAB device. They are used to measure the action of a force from the axle pressures of a moving train load in a particular layer (force measurement, accelerometers, etc.). Sensors are also currently being prepared in the same positions (from S1 to S4) of the upper parts of the railway superstructure. Subsequently, the values of measurements during the movement of train sets at 60, 100, 120, and 160 km/h will be compared, and the resulting degradation will be detected using the KRAB device. So far, sections have been prepared in the transition zone of the bridge structure with the track bed, but other sections are being prepared at the locations of the so-called “jump” from the fixed track to the track bed. The problem of regular tamping with automatic tamping machines (ballasted track) occurs, but further experimental sections are being prepared at the transition point to the fixed railway track according to the curves in Section 3.2 in accordance with monitoring.

3.1. Bridge Objects—Ballasted Track

First, we discuss the bridge object with adjacent parts of track sections, of which the measured LHD values of rails R/L are shown in Figure 2. Markings “ZA1”, “ZA2”, “BA1”, and “BA2” depict the directions of the location of the measured transit zones near the objects (bridge, tunnel), e.g., ZA—towards the city of Zilina; BA—towards the city of Bratislava; “1” and “2” are track numbers. In the figure, the transition zone is in the railway substructure (bridge), where the railway’s upper structure forms a classic ballast bed. The “arrows” in the figures represent the prevailing direction of the trains (approximately 80%). We can conclude that the results show characteristic deflections in the track geometry after the train sets pass through the track.

The figure shows the bridge object and the positions of the sensors for measuring forces from S1 to S4 under the R rail (they are not the subject of this manuscript and are being prepared for subsequent physical measurements in 2025, using monitoring methods other than the KRAB trolley).

The lower parts of all figures with LHD curves present the composition of the structural parts of a particular transition zone (e.g., bridge, tunnel, portal, ballast, FRT, FRT-block, tub, etc.).

The height deformation of rail R in Figure 2b was more significant than that of rail L in Figure 2a because it was the outermost rail R on the slope of this embankment. Although the gravel bed decreased to a minimal extent due to running away into the hill, the track met the criteria for operation. Upcoming measuring sensors S1 to S4 were also located under this rail R, which caused small height shifts in these places. After the next tamping of the track with tamping machines, the heights would be levelled with the L rail in Figure 2a.

The resulting deflections of the LHD height geometry of the R/L rails are evident in the details of this bridge object in Figure 3. We note that the height waves at the LHD in the continuous track bed were not as significant as at the ends of the fixed carriageway. This was because a layer of the track ballast dampened the sudden transition change in the railway substructure to the bridge object. As a rule, the track bed was tamped with tamping machines to the required prescribed geometric position and heights of the track (in contrast to a fixed railway track).

Regular tamping works with automatic tamping machines (to the prescribed, designed position and heights) but does not allow for the monitoring of the time course of the LHD with the KRAB trolley and similar devices. Evaluations can only be carried out at intervals between two tampings of the track bed, as tamping returns the heights to the same prescribed height position.

This was confirmed by measurements on the fixed railway track near the tunnels and the classic ballasted track (Section 3.3). A railway line with a classic construction was generally tamped to the projected height (or optimised / prescribed), according to the criteria for evaluating the operational condition in STN 736360-2 [16] for track monitoring, unlike FRT, where tamping works were not possible. These were operational deviations for the AL level—monitoring limit; IL—intervention or repair limit; and the IAL limit deviation—the limit of immediate intervention in the track.

Shocks and shock waves are expected at construction sites under the track bed in the railway subgrade. The track will more easily guide the vehicle over these objects in places of stiffness change. This applies if the track is correctly and qualitatively built without subsequent deformations. The FRT demonstrates long-term stability; only when the FRT is affected by a change in the stiffness of the lower track structure does it adjust in height, i.e., it deforms.

3.2. Transition Zones—Fixed Railway Track

The following results of the experimental measurements when changing a track with FRT to a track with a track bed point to problems when changing the stiffness of the railway infrastructure. In these places, one structure was interrupted, and there were significant shocks from the wheels of train sets, e.g., Figure 4, Figure 5, Figure 6 and Figure 7.

For comparison, measurements are presented from the monitoring of a direct transition (at the point of change in track stiffness) from a fixed railway track to a track with a railway bed, for example, sections near the Turecky Vrch tunnel near the city of Trencianske Bohuslavice, according to Figure 4, or near the bridge in the town of Trencin, in Figure 7. Reinforced concrete tubs for the track bed were designed here as transition zones. The zero point “0” represents this transition (“jump”) where structural layers and objects were deformed; the resulting values are R/L rail geometry position LHDs.

Based on the results of long-term experimental measurements, we can conclude that the transition zone with its beginnings and ends of the fixed railway track (the ends of the FRT), where there was a transition from a reinforced concrete slab to the track bed, or the end of a bridge structure without a track bed, caused more significant problems on the railway line. In contrast to the FRT, in the transition zone from the gravel track bed (near the FRT), e.g., at a bridge object located at the railway bottom of the track, the track could be tamped regularly with automatic machine tampers (provided that the structural layers of the railway’s lower substructure under the transition zone can affect their deformations, i.e., the TZ).

During this transition at the end of the FRT structure, a “vertical cut” occurred in the longitudinal profile of the track, causing a significant change in its stiffness. This start or end of the FRT at the zero point “0” (approximately above 1 m in the track bed) was tamped poorly. For this reason, “jumps” (deformation waves) arose, which were created by the forces of the wheels of train sets at these points.

The research shows that the most significant height wave of the change in the geometry of the R/L rails occurred at the transition point, and in the case of the constructed reinforced concrete tub, also at its end (schemes—lower parts of figures), but with approximately half the value of the height/depth of the wave (“double jump” height curve, in this case; the length of the tub was 20 m). When the trains travelled in the opposite direction, the material at this point was pushed under the fixed track or the transitional reinforced concrete tub, or the structural layers of materials under the concrete objects or the reinforced concrete tub of the transition zone were deformed.

These longitudinal height deformations (LHDs) of the R/L rails are significant (higher values) mainly because they represent places on the railway track where there is a transition between the reinforced concrete slab (FRT) and the railway ballast. For this reason, research is being carried out on these sections with a change in the stiffness of the railway track. In addition, according to Figure 4, we assumed changes in the heights of the underlying layers, as it is a high railway embankment of 6–8 m, and the said section has experienced problems during the entire period of operation of the railway line. According to this figure, we can also state that the first-stage measurements before and after the track is put into operation are curves that indicate the point “0”—the end of the reinforced concrete block FRT (its fall and subsequent extrusion of the material).

Even if these height waves of the geometric position of the R/L rails (LHD) are detected, solved, and analysed, the track in the given section meets the criteria of operational deviations. As an illustration, the resulting graphs from measurements using the continuous KRAB trolley with all its parameters are shown in Figure 8. The primary task of the research was to reduce the effects of shocks from the wheels of train sets, vibrations, deformations, etc., on objects of track construction, including the stress and wear of structural parts of train sets, as well as to increase the comfort of driving on the track (people and goods). If these deformations are not considered, the geometric position and heights may collapse at the point of change in track stiffness.

These limit values are differentiated by colour for the specific level of intervention in the track, whereby AL level—monitoring limit (green); IL—intervention or repair limit (blue); and the IAL limit deviation—the limit of immediate intervention in the track (red). The limits were set according to the importance of the track, e.g., the European railway corridor with a track speed of 160 km/h.

3.3. Tunnel Portals—Ballasted Track

Other track stiffness changes, such as tunnel portals, behaved similarly to the vertical deformation deflections of the track geometry.

Data were monitored in the measured research section of the transition zone near the portal of the Milochov tunnel, according to Figure 9 (first measurement 9/2023), where there were height changes on the R/L rails. Since it is a classic construction of a railway line with a track bed, the section was tamped with automatic tamping machines to the prescribed geometric position and height of the track according to the AL, IL, and IAL criteria, in line with the standard STN 736360-2 [16].

3.4. Tunnel Portal—Fixed Railway Track

In the case of tunnels with a fixed railway track (FRT), the monitoring process registered the height changes in the wave of the geometrical position of the track (tunnel portals), where it was proven that there was a pushing of the material into the part of the track with a softer stiffness, followed by the subsequent pushing out of the material of the structural layers at the zero point “0” of change. There was also an uplift of the track geometry at the beginning of the reinforced concrete track—Figure 10b.

The positive deformation arose because it was a place with a sudden change in the stiffness of the railway infrastructure. In this case, as shown in Figure 10b, the train pushed a pressure wave that hit the reinforced concrete slab. In front of the tunnel, there was still a terminal block of the fixed railway line FRT-block with a length of 5.08 m, on which there was still an FRT. Therefore, the change in stiffness was shifted up to this point, where the tub–ballast track bed began. Figure 10a depicts a continuous FRT. Since it shows the long-term effect of pressure wave forces on the object, there were elevations of these ends of the FRT, which was proven by all the figures in the paper.

The driving of various vehicles, which enter the tunnel from the road network in this location, during tunnel maintenance or the loading of parts and machines at the tunnel portals, could contribute to the deformations to a certain extent. Alternatively, there can be a double transition zone, i.e., the end of the fixed carriageway and the associated tunnel portal (two transition zones in a row in Figure 10b).

Experimental measurements were carried out using different structural combinations of the railway tracks at the tunnel portals. The results with a continuous track bed were presented in this paper; however, the results regarding the railway track formed by the FRT-reinforced concrete track or slabs are more interesting. In the case of continuous FRT construction at the portal site, it was influenced by this tunnel portal (its massive construction), for example, by the FRT in front of the portal, or vice versa, by the fall or lift of the portal. For a long time, research was mainly carried out for portals with a sudden structural change in the stiffness of the carriageway, where the FRT ends (designed only in the tunnel tube). In these cases, the proposed reinforced concrete tub for the track bed is located before the tunnel portal. These transition zones have a proposed reinforced concrete block glued to the foundations of the portal, and here, a “cut” occurs between these objects; for example, they are presented in [12,13,14,15].

4. Discussion

Sophisticated algorithms and mathematical formulas are part of developing the KRAB measuring device [11]. This device has multiple sensors for detecting various measurements, from which continuously measured results are processed in a solution developed by the KRAB manufacturing company. The results are the values of the measured data (not only presented in this paper but also according to other criteria). Our research processes these numerical data by monitoring and evaluating them specifically for the transition zones of railway infrastructure.

Based on the mentioned long-term observations of the transition zones, it is possible to state certain characteristic basic features of the behaviour of the geometric position of the R/L rails. Differences arise in transition zones with a fixed track (FRT), for example, with a reinforced concrete tub, a track with only a track bed and built-in bridge objects, tunnel portals, culverts in the railway lower substructure, etc. We have to keep in mind that the track bed (unlike the reinforced concrete track with FRT) is tamped regularly based on design or operational criteria, and it is difficult to determine the behaviour of the objects below it because they have the final adjustment of the geometric position and heights of the track made by machine tampers. On the contrary, the track with FRT cannot be tamped; modification is impossible or exceptional (change in pads under the rails with different thicknesses, breaking the reinforced concrete slabs and building a new one), but they have long-term sustainability of the geometric position of the track. The problem is the contact zones in the case of a sudden change in the stiffness of the track of these two systems.

The result of the analysis was the finding that when the railway track stiffness suddenly changed when moving from a reinforced concrete slab to a gravel bed at the zero point “0.0 m”, there were drops of −5 mm and rises of +5 mm (a total of 10 mm over a short distance of 5 to 7 m). The results are evident in Figure 4, Figure 5, Figure 6, Figure 7 and Figure 10b. Although the vertical deformation was okay from the point of view of a railway line in operation (they met the criteria Al, IL, and IAL), this place was dangerous because the shock wave from the train set, which gradually deformed the entire geometry of the track in the longitudinal direction, including its objects, hit a solid reinforced concrete slab, or vice versa—jumped from it into the gravel. These were the strong forces that were acting on the embedded material in the track. Another wave was created at the end of the reinforced concrete tub with gravel in the point “20.0 m”, at approximately 2 to 4 mm, where the reinforced concrete tub slab ended in the substructure.

This paper describes measurements in transition zones and on a classic gravel track, where it was not possible to demonstrate these deformations of the track geometry, as the railway track was regularly tamped according to the AL, IL, and IAL criteria, e.g., Figure 2 and Figure 9 (it could only be detected during a very long time interval of non-tamping on the track).

After the construction of the transition zones was put into operation, the material was degraded into the following 1–4 stage measurements. Then, stabilisation occurs with some small degradation process. Changes in heights (and LHD height deflections) over time are evident from the legends of all figures where MONTH/YEAR is indicated, for example, as 5/2014, 10/2014, 4/2015, … 10/2021, 3/2022, 12/2022 in figures, etc. The gradual degradation over time from the measurements when the transition zones were put into operation can be seen in the figures.

Through the monitoring and inspection of the railway infrastructure, it was proven that there were shocks, vibrations, jumps, and a pushing of the materials of the structural layers in the transition zones, according to the diagrams in Figure 11.

Figure 11 summarises all the detected characteristics on the track with FRT (Figure 11a–c). However, if the track without FRT is only with a track bed, only the undulation of the geometric position of the track heights is presented (Figure 11d). In this case, there was no change in track stiffness directly under the rails and sleepers (continuous track bed) because the track was tamped using tampers.

With a continuous track bed, it was not possible to determine the behaviour of the geometric position and height of the track depending on the built-in object under the track bed, as even here, the track was tamped regularly or at prescribed AL, IL, or IAL intervals. The geometry of the track was characterised by height waviness in small height changes of ±2 mm. We must note that if a bridge, tunnel portal, or culvert has a continuous track bed, we could not record it, for example, with the tunnel portal in Figure 9 or Figure 11d. Indeed, if the track was not tamped, its deformations would be recorded. If the continuous track bed ended at a bridge abutment or a tunnel portal, we considered it a classic transition zone, like the end of a fixed railway track.

Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10 provide information about the measured data displayed by the curves. Figure 11a–d summarise the general properties of this monitoring process. Figure 12a–c supplement them with photos from the measured research sections on the railway.

All the presented research tasks of the Department of Railway Engineering are carried out to verify the long-term behaviour of the geometric position and heights of the tracks in each section of various transition zones. The tasks are carried out within the framework of track exclusions, organised in cooperation with the Railways of the Slovak Republic—the Infrastructure Manager and the Railway Research and Development Institute (VVUZ).

The presented practical implementations will be used by designers in the field of railway infrastructure in the modernization of the European railway corridors in the Slovak Republic for future designs of transition zones, for new designs of transition zones, and also for the reconstruction of existing ones. In places of transition zones with a sudden change in stiffness between solid reinforced concrete railway tracks and track beds, their adjustment is difficult in the reinforced concrete slab, and poor tamping with tamping machines of the track bed is also observed at this location—zero point “0” (there is a vertical shear between the materials). This means that track geometry and height control are also necessary to prevent train accidents.

Because in the Slovak Republic, this issue is not sufficiently addressed in the relevant legislative documents, it is necessary to proceed with their amendment in connection with the long-term experience of advanced railway administrations and to incorporate European standards into the proposals of designers in railway infrastructure.

Several years of track modernisation experience have shown that increased investments in the quality of transition zones can increase driving comfort and the life of the track, as well as reducing the costs of its maintenance and repairs (by increasing the time intervals between the necessary AL, IL, or IAL interventions).

The current degradation of the geometric position and its heights indicates that something is happening at the critical points in the substructure of the railway body. Another option is according to the maximum and minimum values of the height deflection curves; for example, sensors will be placed to measure the acting forces, shocks, and vibrations in a certain layer. These are invisible defects, and it is necessary to start removing them as part of reconstruction or modernisation. Designers can also design new types of transition zone parts, for example, by uncovering objects or using various non-destructive methods to detect them.

The research has a new direction [17], i.e., the selection of experimental sections of transition zones of various types with a measurement of effects by force sensors up to 200 kN, accelerators, and vertical displacement sensors during the passage of train sets at various speeds up to 160 km/h (passenger and freight trains). The sensors will be placed throughout the railway body in the transition zone, namely in the upper section, by increasing the speeds of the railway substructure. The research will mainly benefit in increasing the speed of the railway infrastructure.

5. Conclusions

Adjusting the stiffness gradation was, is, and will be necessary at the transition points between the classic construction of the railway’s upper structure and the construction of the so-called fixed reinforced concrete roadway (FRT) or ballast-less superstructure, as well as in the case of constructions of the body of the railway substructure.

Based on experimental measurements of selected test sections established on the European railway corridors, the current options for solving transition zones must be optimised using numerical modelling, or new solutions must be proposed. From this aspect, these solutions must not only respect the experiences gained from the research of the given issue but also accept new experiences and perspectives on the solution of transition zones according to advanced railway administrations.

At the same time, the effort will be to apply, to the maximum extent possible, innovative structural materials and elements in the construction process in these places of the transition zones, not only in the part of the construction of the railway superstructure but also in the railway substructure (different types and shapes of structural layers and their different materials, reinforced concrete plates and wedges, small pilots, etc.). From this point of view, the expected benefit will be the differentiation of structural materials and elements and their influence on achieving a numerically optimal gradation of roadway stiffness between structures with different stiffnesses, which occur in large quantities on every railway infrastructure.

A relevant contribution will be, in addition to the optimisation of the applied current transition zones, their modification, i.e., the preparation of structurally and materially new constructions of transition zones for different areas of different stiffness parts on modernised railway infrastructure lines, respecting actual operating conditions (track speeds, type, and number of trains) and also various parts of construction objects of the underlying structure (bridges, tunnels, culverts, etc.). At increased speed, the running of trains along these transition zones and their subsequent track sections will be more reliable and safer with increased resistance to their deformations by reducing the effects of the forces acting on the given places. Numerical modelling of these elements and the forces acting between them (from the chassis wheels through the R/L rails to all layers of the railway body) is a subsequent step of the new project for the years 2025–2028 [17].

Research fills gaps in railway practice, as it is necessary to monitor many transition zones. Based on the detected longitudinal height deflections, designers can propose new designs to eliminate deficiencies in their design and construction and, above all, to ensure the long-term sustainability of the track geometry in these critical railway positions.

Research mainly contributes to designers who design railway infrastructure that can improve transition zones in their design or to railway engineers in the context of the reconstruction and modernisation of railway infrastructure.

From the point of view of a passenger on a passenger train and also of goods being transported on freight trains, these places with a sudden change in the stiffness of the running track (iron–concrete on the track bed) are very unpleasant. Undesirable horizontal and vertical forces, including shocks and vibrations, occur. In this section, the driving comfort is disturbed, especially at high speeds on the track. In the Slovak Republic (and in other countries), there is mixed train traffic (passenger and freight), and forces from heavy, long, and old freight trains act very unfavourably on these sections of tracks. The struggle with the consequences of these forces begins with a high-quality design proposal from designers during the construction of railway lines and continues during their operation.