## 5 Results
Rail corrugation monitoring is performed by computing the RL within the four wavelength bands prescribed by EN 13231-3 [26], which defines the acceptance criteria for the grinding operations on the rail running surface. The wavelength bands are 10-30 mm, 30-100 mm, 100-300 mm and 300-1000 mm. Attention has been paid only to the 30-100 mm and 100-300 mm wavelength bands, where corrugation is typically found in the specific line. Figure 10: Speed profile and moving rms of the vertical acceleration measured at the axleboxes of the leading wheelset of the instrumented bogie: (**a**) day 1; (**b**) day 7. The RLs in 100-300 mm and 30-100 mm bands are shown for the considered runs in Figures 12 and 13, respectively. The curves of the track section under analysis are highlighted in the figures by vertical dashed lines for a better understanding. By looking at the RL in the 100-300 mm wavelength band, the highest RL values are observed on the low rail of the curves (Figure 12b for the second curve). This is in accordance with the spectrograms of Figure 11: they indicate a corrugation wavelength of 130 mm on the first curve and a 170 mm corrugation wavelength on the second one. The average speed on the first right curve is about 33 km/h, while the second one is negotiated at an average speed of 43 km/h (Figure 10). Thus, for both the curves, the frequency of excitation by the rail irregularity is about 70 Hz according to Equation (3), which corresponds to the natural frequency of the first bending mode of the wheelset. On account of the differences in the transfer functions reported in Figure 9 at the same frequency, the RL on the low rail estimated with the flexible wheelset model is 7 dB higher than the one predicted with the point mass model. The same difference is observed both on Day 1 and Day 7. This indicates that the transfer function calculated with the flexible  
Figure 11: Spectrogram of the rail irregularity on Day 1, along with the vehicle speed (solid line) and the horizontal curvature of the track (dashed line): (**a**) left rail; (**b**) right rail. Figure 12: Roughness Level in the 100–300 mm wavelength band: (**a**) left rail; (**b**) right rail. wheelset model is more sensible for finding the presence of rail corrugation and therefore the proposed approach will be more effective for its monitoring. The RL on the high rail is lower with respect to the low rail, but its absolute value is in any case significant. As mentioned before, this is related to the fact that the two axleboxes of the wheelset are not dynamically decoupled, and thus both axlebox accelerometers are measuring vibrations at the excitation frequency defined by the corrugation wavelength. The low rail of the second left curve shows a significant increment in the RL of about 5 dB from Day 1 to Day 7, indicating that the corrugation growth rate is high and attention should be paid in the scheduling of rail grinding operations. Concerning the 30-100 mm wavelength band, the RL on the two rails shows a minor variation in the considered time interval. This is consistent with the fact that no corrugation is observed within this wavelength band. As anticipated in Section 3.3, the transfer function of the measurement system using the point mass model overestimates the rail irregularity in the medium-to-high frequency range, expected at the wheelset natural frequencies (Figure 9). The 30-100 mm wavelength band corresponds to the 90-300 Hz frequency range for the first curve (negotiated at 33 km/h) and to the 120-400 Hz range for the second one (negotiated at 43 km/h).