## 6 Conclusions  
This study analyzed rail fracture data based on the status of the urban rail track infrastructure, applied machine learning, and conducted a CFD analysis to investigate the track-installation conditions contributing to rail fractures near ventilation shafts. Key parameters influencing CWR weld fractures were identified, and the temperature influence range from ventilation shafts was analyzed. In addition, a validated numerical analysis model incorporating temperature variation as a load condition was used for analyzing the axial force during CWR fractures. The CWR fractures predominantly occurred in winter, from October to March, and the tunnel temperature analysis determined the optimal median temperature for urban rail systems to be 13 \({}^{\circ}\)C. The Gaussian probability density analysis showed that the rapid increase in the CWR axial tensile force could lead to weld fractures when temperatures dropped sharply by 35 \({}^{\circ}\)C in extreme cold conditions near ventilation shafts.  
A machine learning analysis of parameters influencing CWR weld fractures near urban rail ventilation shafts confirmed that the model appropriately reflected actual CWR fracture site conditions, verifying the validity of the machine learning model. The final key parameters (temperature variation at fracture occurrence, distance from ventilation shaft, track geometry conditions, etc.) were identified and used in CFD and FEA. The CFD analysis results showed that the temperature influence on the track structure was most significant within \(\pm\)10 m (20 m) of the ventilation shaft center. The maximum influence range extended up to \(\pm\) 20 m (40 m), aligning with field investigation findings. The axial force analysis of rail fractures caused by temperature variations around urban rail ventilation shafts indicated that axial stress at the CWR weld fracture point was \(\sim\)71 MPa, which is 23% lower than the CWR base material threshold (92 MPa).  
Korean urban railways operate standardized welding procedures and inspection systems, but the microstructure vulnerability of the heat-affected zone (HAZ) exists as a structural limitation. It is believed that improving the maintenance management of rail welding, such as welding cooling rate control and stress relief heat treatment, rather than tunnel temperature management, would be a more realistic response.  
Based on this study, the following suggestions are made to ensure the maintenance and safety of CWRs.  
1. CWR installation guidelines: 1. Avoid extreme cold or extreme heat during construction of the track; 2. The location of the CWR field welding part (thermite welding, etc.) should be located \(\pm\)20.0 m away from the center of the ventilation shaft.  
Figure 18: Axial stress vs. rail fracture temperature variation.  
Maintenance guidelines: (1) Avoid extreme cold or extreme heat when performing rail welding work in a tunnel during operation; (2) When performing field welding work, perform the work within the medium temperature range (urban railway 13 \(\pm\) 5 \({}^{\circ}\)C) if possible.  
Conceptualization, J.-Y.C. and J.-S.C.; methodology, S.-Y.P.; formal analysis, J.-Y.C. and S.-H.K.; investigation, S.-Y.P. and J.-S.C.; data curation, S.-Y.P. and S.-H.K.; writing--original draft preparation J.-Y.C. and S.-H.K.; writing--review and editing, S.-H.K. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.  
The authors declare no conflicts of interest.