Influence of Rail–Track Structural Irregularities on Train-Induced Ballast Settlement

1. Introduction

Numerous experimental and numerical studies have been conducted in recent decades to assess how railroad system irregularities affect track and ballast settlement caused by trains and how this in turn affects passenger comfort. Sadeghi et al. sought to determine how rail defects affect the comfort of trains running on slab tracks. A numerical model was created to simulate the interaction between the car and the slab track. According to the research, travel comfort on slab tracks is greatly impacted by rail irregularities with short wavelengths. In particular, it was discovered that ride comfort significantly decreases when the wavelength of rail irregularities falls below 0.75 m.

Fallah Nafari et al. investigated how track stiffness affects the rail. The results showed that variations in track stiffness significantly impact the bending moment of the rail. Additionally, it was discovered that these variations result in higher wheel–rail contact forces, which in turn cause uneven wear on the rail head and the development of rail corrugation.

Bian et al. integrated track irregularities in a 2.5-dimensional track–ground model and found that amplitude mainly affects low-speed vertical responses, while short-wavelength irregularities increase vibrations in both low- and high-speed trains, with wavelength having little effect on far-field ground vibrations at high speed. Short-wavelength irregularities produce high-frequency track vibrations, while long wavelengths are dominated by axle loads.

Most previous numerical studies have simplified the rail–track system by modeling rails as ideal line elements without considering key structural details such as sleepers and fasteners. Additionally, research that addresses track irregularities is limited, and studies that investigate combined irregularities affecting both the rail and structural components are particularly scarce. To fill this gap, three-dimensional train–track models were developed in this study to examine the effects of track irregularities on ground vibrations and ballast settlement, considering various types of irregularities and train speeds.

2. Methodology

In this section, the methodology for numerically modeling the influence of train–track irregularities on ballast deformation under varying train velocities is presented. The simulations were conducted in PLAXIS 3D, an advanced geotechnical tool capable of capturing the complex interactions between moving loads and rail infrastructure.

To systematically examine the effects of irregularities and speed, three three-dimensional models were developed—one representing an ideal track and two incorporating common irregularity scenarios (irregularities in the rail only and in both the rail and sleepers). Train speeds of 100, 200, and 300 km/h were simulated to evaluate their dynamic impact on ballast behavior.

2.1 Description of the Finite Element Model

A three-dimensional finite element model of a rail–track system under moving train loads was developed and validated. The analysis was conducted in two phases: static and dynamic. In the static phase, the boundary conditions and stability of the soil–track model were verified. In the dynamic phase, the influence of soil mass inertia forces on the interaction between moving train loads and the subgrade soil was considered.

The subgrade soil model was 180 m × 100 m in plan and reached a depth of 30 m. 10-node solid elements were used to discretize the track system and subgrade soil. The subgrade soil, ballast, and sleepers were modeled as volume elements using bilinear elastic–plastic material following the Mohr-Coulomb failure criterion to achieve higher accuracy. The rail was modeled as a linear elastic material with a Young's modulus of E = 206,000 MPa.

2.2 Introduction of Irregularities in the Rail–track System

In practice, wear, plastic deformation, and fatigue induced by repetitive train loads are the primary causes of rail irregularities. Variations in rail geometry may also arise from improper installation, uneven sleeper support, or thermal expansion effects. Likewise, cracking, settlement, and deterioration of sleepers due to heavy loads, aging, or insufficient maintenance can introduce additional irregularities.

Two common types of irregularities were simulated in this study. In the first case, an irregularity was introduced only in the rail. In the second case, both rail irregularity and sleeper removal were simulated at the same location to represent another frequently encountered field condition.

3. Results and Discussion

Three different rail–track configurations were analyzed: (a) an ideal track without irregularities, (b) a track with irregularities only in the rail, and (c) a track with irregularities in both the rail and sleepers.

The results clearly show the significant influence of rail irregularities on the deformation behavior of the track system. These irregularities disrupt the uniform load distribution along the rail, producing localized stress concentrations and leading to higher settlements. Specifically, at 100 km/h, the maximum settlement increased from 4.5 mm in the ideal case to 11.12 mm in the presence of rail irregularities; at 200 km/h, from 4.7 mm to 11.52 mm; and at 300 km/h, from 4.6 mm to 11.41 mm.

At higher speeds, irregularities become more critical as they amplify dynamic effects through vibration and impact loading. Higher speeds increase inertial and impact forces, causing irregularities to excite stronger vibrations in the track.

Deformation increases even further when irregularities are present in both the rail and sleepers. Ballast compression is accelerated and stress concentrations are increased when rail connection and sleeper support are lost simultaneously. The maximum settlement rose to 15.23 mm at 300 km/h, representing a nearly threefold increase compared to the ideal condition. This suggests that irregularities in the sleeper layer are just as problematic as those in the rail and that their existence magnifies each other.

4. Conclusion

This study investigated the influence of rail and sleeper irregularities on the deformation behavior of rail–track systems under varying train speeds using advanced three-dimensional finite element modeling in PLAXIS 3D. The results clearly show that even minor defects significantly degrade track performance by disturbing uniform load transfer, producing localized stress concentrations, and accelerating ballast settlement.

When irregularities affected both rails and sleepers simultaneously, settlement increased to about 15 mm, representing nearly a threefold rise compared to the ideal condition. Train speed was identified as a major amplifying factor, where higher speeds intensify inertial forces, vibration, and impact loading.

Tracks with irregularities—particularly combined rail–sleeper defects—should be treated as high-risk sections requiring prioritized maintenance and monitoring, especially on lines operating at 200–300 km/h. Regular rail grinding, alignment corrections, sleeper support restoration, and stiffness-control measures such as ballast renewal or under-sleeper pads can effectively reduce settlement progression.

Overall, this research demonstrates that maintaining geometric accuracy and structural continuity in rail–track components is critical for prolonging service life, enhancing safety, and reducing maintenance costs, particularly in modern high-speed railway networks.