## 3 Field Test and Results  
Differences in measured values were compared according to changes in the number of sensors by measuring the field track using the previously calculated optimal sensor positions. The rail surface roughness measuring system used for on-site measurement was the Auto Rail Checker (ARCer), which allows control of speed and automatic measurement, as shown in Figure 5. The ARCer used for measurement also features a brushless motor with an incremental encoder to measure distance, three LVDT sensors installed at movable positions, Li-Ion battery, and wireless data transmission functionality. The probe tip of the LVDT has been modified as shown in Figure 5 and curvature processing was applied as defined in the EN 15610:2009 standard [9]. The ARCer also has a platform with a wheelbase of 716 mm and front and rear driving wheels with diameters of 100 mm. The speed at the time of measurement was 0.5m/s. As shown in Figure 5, the ARCer is equipped with three sensors (S1, S2, and S3), and the sensor positions (\(d_{1},d_{2}\), and \(d_{3}\)) are adjustable according to the measurement algorithm. It was used to measure rail surface roughness at curved section where corrugation was well developed along Seoul Metro lines. Two track locations with different corrugation wavelengths, section A and section B, were choosen for experiment. Each sensor position was used to repeatedly measure the same section using the optimal position for the number of sensors described above. Measurement and analysis showed that longitudinal rail surface roughness conformed to the EN 15610:2009 standard [9]. The subject of the field test was a curved section of a concrete sleeper track with a well-developing corrugation (\(\lambda\) =350 mm), as shown in Figure 6. Three sections of the curved line were repeatedly measured with three sensors, two sensors, and one sensor using ARCer at 10-meter intervals. The optimal sensor positions used in the field experiment were \(d_{1}\) = 222 mm, \(d_{2}\) = 327 mm, and \(d_{3}\) = 432 mm for three sensors and \(d_{1}\) = 255 mm and \(d_{2}\) = 380 mm for two sensors for comparison. When using one sensor, the center position of \(d_{1}\) = 327 mm was used. The roughness spectrum analysis was based on the displacement values of 1-9 m, excluding before and after the 1 meter measurement, which was the acceleration/deceleration section, based on the displacement values measured from each sensor. Figures 7 and 8 show the comparison of the rail surface acoustic roughness levels obtained through the measured acoustic roughness values and frequency analysis for each number combination and position of the sensors. In the field test, measurements of two sections were compared for different points on the same track. The measured corrugation in the second section B rather than the first section A shows a broad wavelength band, as shown in Figure 8.  
Figure 4: Comparison of magnitude of transfer function according to the number of sensors used; (**a**) Overall magnitude of the transfer function; (**b**) Magnitude of transfer function at specific wavelength range from 20 mm to 80 mmAlthough corrugation with a wavelength of 30-35 mm confirmed in Figure 8 appears regardless of the number of sensors used, using one sensor amplified the corrugation section, or the acoustic roughness level value showed a significant difference of at least 10 dB at the \(\lambda\) = 0.314 m wavelength, which was related to wheel diameter. Measurements using two sensors were lower than those using three sensors at relatively short wavelength regions. Figure 5: Auto Rail Checker (ARCer) with multiple displacement sensors  
Figure 6: Field \(\mid\) measurement of corrugated rail on curved section.