## 3 Noise Characteristics According to Vehicle Maintenance Conditions  
This study aims to identify the effect of vehicle maintenance conditions on abnormal interior noise by performing interior noise measurements under different maintenance conditions. International standards for noise measurement include ISO 3381, GB/T 3449-2011, and JIS E 4021-2008. In this study, noise measurements were conducted in accordance with KS I ISO 3381, which is identical to the widely adopted ISO 3381 [20; 21; 22; 23]. As shown in Figure 4, noise sensors were installed inside the vehicle, and the noise recorded during operation was stored using the data storage device shown in Figure 4. Four test vehicles were selected, each reflecting different maintenance conditions, as shown in Table 3. Interior noise levels were measured during operation for each case, and equivalent noise levels were analyzed for segments between stations. Figure 5 and Table 4 present the equivalent interior noise levels measured in each section under all vehicle maintenance conditions. Overall, the noise levels in sections with rail corrugation were significantly high, showing a similar trend across the different maintenance conditions of the vehicles. In Case 1, which had the poorest maintenance condition, the average noise level across all sections was 80.6 dB, with a maximum value of 89.2 dB. In contrast, the vehicle in Case 2, which completed a major overhaul, exhibited an average noise level of 79.9 dB and a maximum of 85.9 dB. The vehicle in Case 3, with only machined wheels, recorded an average noise level of 79.6 dB and a maximum of 86.0 dB. Compared to Case 1, Cases  
Figure 4: Interior noise measurement equipment. \begin{table} \begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Case**} & **Year of Vehicle** & **Wheel** & **Wheel** & **Remark** \\  & **Manufacture** & **Diameter (mm)** & **Profile State** & **Remark** \\ \hline
1 & 2012 & 621 & worn & before heavy maintenance \\ \hline
2 & 2012 & 660 & new & after heavy maintenance \\ \hline
3 & 2012 & 618 & new & after wheel reprofiling \\ \hline
4 & 2022 & 660 & moderate & new vehicle \\ \hline \hline \end{tabular}  (1) Case 1: Vehicle just before a major overhaul, conducted every 4 years, with a wheel diameter reduced to 621 mm from the original 660 mm and with worn wheel treads. (2) Case 2: The same vehicle as in Case 1, but after a major overhaul, with new wheels installed, a wheel diameter of 660 mm, and freshly machined wheel treads. (3) Case 3: Vehicle with wheels machined due to damage during operation, with a wheel diameter of 618 mm and machined wheel treads in their original condition. (4) Case 4: Recently manufactured vehicle with a wheel diameter of 660 mm and moderately worn wheel treads after accumulating approximately 50,000 km of service. Figure 5 and Table 4 present the equivalent interior noise levels measured in each section under all vehicle maintenance conditions. Overall, the noise levels in sections with rail corrugation were significantly high, showing a similar trend across the different maintenance conditions of the vehicles. In Case 1, which had the poorest maintenance condition, the average noise level across all sections was 80.6 dB, with a maximum value of 89.2 dB. In contrast, the vehicle in Case 2, which completed a major overhaul, exhibited an average noise level of 79.9 dB and a maximum of 85.9 dB. The vehicle in Case 3, with only machined wheels, recorded an average noise level of 79.6 dB and a maximum of 86.0 dB. Compared to Case 1, Cases2 and 3 showed a slight reduction of approximately 1.0 dB in the average noise levels; however, the maximum values decreased by more than 3 dB. These findings indicate that wheel machining contributes to a reduction in interior noise. Comparing Cases 2 and 3, the noise levels were nearly identical, suggesting that other inspection items during the major overhaul, aside from the wheel machining, had little effect on the interior noise during operation.