## 3 Results of Interior Noise  
As shown in Figure 4, the interior noise of the train in the static state was measured when the train was stopping. The equivalent A-level of vehicle interior noise is 69.9 dB(A) when the vehicle is stationary. It is much lower than the limit value of the metro vehicle interior noise specified in the standard [30], which is 83 dB(A). There is no obvious peak frequency, which eliminates the influence of the noise of vehicle parts and auxiliary equipment on the internal noise of train operation. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sampling Interval** & **Measuring Speed** & **Precision** & **Weight** \\ \hline
1 or 2 mm & 3–4 km/h & 0.01 μm & 8 kg \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Detailed parameters of CAT. Figure 3: Rail corrugation condition. Figure 2: Corrugation Analysis Trolley. The noise measurement results in the station are shown in Figure 5. The noise in the station increases from 59.7 dB(A) to 85.8 dB(A) when the train arrives at the station. The noise has no obvious peak when the train does not arrive at the station, which eliminates the possible influence of other abnormal noise sources in the station. The metro tunnel resulted in a high absorption at a low frequency. When the train arrives at the station, the noise at the frequencies from 200-2500 Hz increases greatly, while there is less of an increase in noise at the low frequency. In order to eliminate the influence of passengers on the noise test results, the total value of the noise of three measuring points in the train during the off-peak period is calculated, and the results are shown in Figure 6. A-H sections refers to the seven sections with the most complaints from passengers. The train has higher speeds in these sections. Figure 4: One-third octave diagram of standing noise. Figure 5: The noise measurement results in the station. Figure 6: Interior noise test results. The measurement result of the interior noise is large, and there are many intervals exceeding the interior noise limit of 83 dB(A). Among them, the noise value of the middle carriage in D-E section reaches 91.8 dB(A). Since it is the same train and the number of passengers on the train does not change significantly, it indicates that the track status in the D-E interval is abnormal. In addition, the sound pressure level of the middle carriage is slightly higher than that of the head and rear carriages, mainly because the metro train is composed of six motor cars and two trailers, the head and rear carriages are trailers, and the middle six carriages are motor cars. Therefore, the noise source of the middle carriage is more than that in the head and rear carriages, resulting in a slightly larger noise value in the middle carriage. In order to analyze the noise test results more effectively, the subsequent noise test and data analysis in this paper all focus on the middle carriage. According to the noise test results, the noise in the vehicle between D-E section is relatively large. In order to discover the reason for the excessive noise in this section, the situation of the section line was investigated. The section adopts the structure of the monolithic track bed with the DTVI2 fastener and has a total length of 1600 m. There are two curved segments and one uphill segment. The tunnel adopts the form of circular section. The inner diameter of the tunnel is 5.4 m and the outer diameter is 6 m. The specific conditions of the section are shown in Figure 7.  
Figure 7: Line situation of D–E section. (**a**) Tunnel section form, (**b**) Line planar graph. The time-varying sound pressure level of the test results in this interval is analyzed, as shown in Figure 3:0 The interior noise increases gradually in 0\(\sim\)15 s, and the train is speeding up at this time.