## 2 Field Measurements
### Measurement Results  
As shown in Figure 4, the results of the dynamic wheel load measurements indicate no clear changes in the wheel loads with train speed because there was no significant difference in the traveling speeds of the trains passing over the measurement section. In addition, the measured wheel load ranges were found to be similar before and after rail pad replacement. As such, the measurement results indicate that rail pad replacement has an insignificant effect on the dynamic wheel load experienced by the track for all evaluated train types. According to the measured dynamic wheel load results for rail pad replacement, the dynamic wheel load before rail pad replacement is similar to that after rail pad replacement, as shown in Figure 5a,b. The rates of all wheel load fluctuations calculated using the dynamic wheel load measurements were greater than zero, as shown in Figure 5c. The results of the dynamic lateral wheel load measurements show that there were only minor changes under different speeds because the different train types traveled through the measurement section at similar speeds, as shown in Figure 6. In addition, the distribution of the measured lateral wheel load was found to be similar before and after rail pad replacement, as shown in Figure 6a,b. This indicates that rail pad replacement has little effect on the dynamic lateral wheel load for each train type. Figure 4: Measured dynamic wheel load by train type. **(a)** Before rail pad replacement and **(b)** after rail pad replacement. Figure 5: Comparison of the measured dynamic wheel load according to rail pad replacement. (**a**) Before rail pad replacement; (**b**) after rail pad replacement; and (**c**) dynamic wheel load fluctuation for before and after rail pad replacements. The measured displacement of the rail and the track girder according to rail pad replacement, as shown in Figure 7, was not significant. As shown in Figure 7, the difference in vertical displacement according to rail pad replacement between the rail and the track girder was approximately 3%-6%, indicating that the rail pad was insignificantly affected by changes in track displacement. This is due to the structural characteristic of the tested track, the relatively hard rail pad (kpad = 120-140 kN/mm), and the rail fastener with a strong fastening force [2]. Therefore, the rail and track girder were coupled with a strong force and exhibited integrated behavior, as there are no other components that provide elasticity between the rail and track girder [2]. The measured vertical rail displacements shown in Figure 8 indicate that there were insignificant changes in the vertical rail displacement under different traveling speeds for all tested train types in the measurement section. In addition, the distribution of vertical rail displacement was found to be similar before and after rail pad replacement, as shown in Figure 8a,b. The measured displacement of the track girder shown in Figure 9 indicate that there were no significant changes in the displacement of the track girder under different traveling speeds for all tested train types in the measurement section. In addition, the distribution of track girder displacements was found to be similar before and after rail pad replacement, as shown in Figure 9a,b. This indicates that for each train type, the impact of rail pad replacement on track girder displacement was insignificant. Figure 6: Measured lateral wheel load by train type. (**a**) Before rail pad replacement and (**b**) after rail pad replacement. Figure 7: Examples of measured vertical displacements of the track girder before and after rail pad replacement. (**a**) AREX and (**b**) KTX. The measured lateral rail displacements and bending stress of the rail shown in Figures 10 and 11 indicate that there were insignificant changes in the dynamic response under different traveling speeds for all tested train types.