## 2 Methods  
A metro line in Beijing was selected to test the interior noise. This line has the characteristics of a fast train operating speed, short departure intervals and a large passenger volume. Moreover, the generation period of rail corrugation is short; the shortest time is 2-3 months. The noise test equipment used in the test included a sound pressure sensor, high-performance hand-held analyzer, tripod, etc. The sound pressure sensor and high-performance, hand-held analyzer are manufactured by China Orient Institute of Noise and Vibration (Beijing, China). Detailed parameters of the noise test equipment are shown in Table 1. The basic principle of the test instrument is to convert the mechanical quantity into electric quantity by the sensor, and then measure and analyze the electric quantity, so as to obtain the measured mechanical vibration quantity. Before measurement, the sound calibrator is put on the sensor to calibrate the whole measurement system with one or more frequencies within the test frequency range. Referring to the Chinese standards, GB/T 3449-2011 [29] and GB/T 14892-2006 [30], the noise in the head, middle and rear carriages was tested, respectively. These standards specify the noise limits, measurement methods and main contents of test reports of urban rail transit trains. According to the standards, the sound pressure sensors were arranged at \begin{table}
\begin{tabular}{c c} \hline \hline
**Instrument Name** & **Parameters** \\ \hline Sound Pressure Sensor & Maximal measuring range: 134 dB; \\  & Sampling frequency: 20–20 kHz; \\  & Working environment: \(-\)35–80 \({}^{\circ}\)C; \\  & Size: 85 mm Length, 12.7 mm radius; \\  & Date of production: December 2017 \\ \hline \hline \end{tabular}
\begin{tabular}{c} Channel number: 4; \\ Channel coupling mode: AC, DC, ICP; \\ Input range: 100 mV, 1 V, 10 V; \\ Sampling frequency: 0–51.2 kHz; \\ Size: 202 mm \(\times\) 120 mm \(\times\) 50 mm; \\ Date of production: September 2018 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Detailed parameters of the noise test equipment. 1.2 m above the front bogie of the train, and there was no other noise source within 1 m. The specific measuring points were shown in Figure 1.  
The interior noise of the vehicle was tested in the static and running condition. The test was conducted during an off-peak period and there were few passengers on the train. During the test, the running speed of the train was 85 km/h. The running gear of the train had no recent out-of-roundness faults, and the wheel surfaces were in good condition. The noise in the station was also measured when the train entered the station. The measuring instruments were placed 2 m away from the platform shield doors. The sound pressure sensor was placed at the train entrance with a height of 1.6 m and there were no passengers nearby. For the noise measurement results, a spectrum analysis was carried out, and the noise equivalent \(A\) sound level was calculated by the following formula:  
\[L_{A}=10\lg\sum_{i=1}^{n}10^{L_{A_{i}}/10} \tag{1}\]  
where \(L_{A}\) is A-weighted sound pressure level, dB(A); \(L_{Ai}\) is the \(i\)-th octave band sound pressure level, dB(A). After the analysis of the interior noise, the section with abnormal noise inside the train was selected to test the rail corrugation. Corrugation Analysis Trolley (CAT) was used in the test. It is manufactured by the Rail Measurement Company (Cambridge, Britain, UK) for measurement and analysis of rail corrugation. The appearance of the instrument is shown in Figure 2, and the detailed parameters are shown in Table 2. The instrument integrates the acceleration sensor on the high hardness steel balls. The irregularity of the contact point is determined by the vertical micro vibration of the steel balls. The sampling interval is 2 mm. During the test for rail corrugation, the instrument was placed on the surface of the rail, and connected to the notebook computer via the data line.