## 4 Case Study
### Case Description  
The Lanzhou-Xinjiang Railway, with a total length of 2423 km, traverses complex and diverse terrains, including the Gobi Desert and alpine valleys. The line is subject to harsh climatic conditions year-round, such as extreme temperature fluctuations and frequent sandstorms. In addition, it plays a critical role in supporting heavy passenger and freight transportation in the region. The combination of long-term, high-frequency train operations and challenging environmental conditions significantly accelerates the wear and tear of track infrastructure. As a result, track maintenance operations along this corridor face exceptionally high safety risks, making it a representative and suitable case study for conducting research on risk assessment in railway track maintenance. In railway track systems, rails are subjected to both vertical loads from train wheels and lateral horizontal forces. Under high-intensity cyclic loading, internal structural defects can develop, such as nuclear flaws, bolt hole cracks, transverse cracks at the rail base [35], and rail weld defects [36]. As these internal defects progress into deep cracks or fractures, they significantly increase the risk of train derailments. Turnouts represent particularly complex structures within the track system. Components such as the point rails and wing rails, especially near the frog gap, endure substantial impact forces from passing trains. These areas are highly prone to wear, deformation, and other structural defects [37], making them common locations for rail failures. Additionally, under repeated train loading, ballast beds may exhibit various forms of abnormal deformation, including bed deformation, ballast contamination, ballast pulverization, ballast collapse, mud pumping, and ballast hardening [38]. To address these issues, maintenance activities such as rail grinding, rail replacement, frog replacement, large machine cleaning, and manual tamping are commonly performed. Accordingly, this study selected these types of maintenance operations as the focus of analysis. Based on the track maintenance records from the Lanzhou-Xinjiang Railway, five maintenance operations were randomly selected as evaluation samples. Detailed information regarding these samples is shown in Figure 4 and Table 4.  
Operation \(A_{1}\) is large machine grinding, primarily aimed at eliminating defects such as rail corrugation, fish-scale cracks, and rail head flow, thereby reducing vibration and noise during train operations. Operation \(A_{2}\) is large machine cleaning, which focuses on addressing track bed defects such as hardening and mud pumping, replenishing clean ballast, and restoring the elasticity of the track structure. Operation \(A_{3}\) involves frog replacement. Due to the presence of cracks in the switch rail, the damaged frog components must be replaced promptly to ensure the safe and normal operation of the turnout area. Operation \(A_{4}\) is manual tamping, intended to eliminate longitudinal level irregularities in the track. By tamping the track, smooth and stable train operation is maintained. Operation \(A_{5}\) is rail replacement, necessitated by serious rail defects such as contact fatigue fractures and shelling. The damaged rail must be replaced in a timely manner to eliminate potential safety hazards. The operation times and locations are detailed in Table 4.