## 4 Results and Discussion
### Track Impact Factor Based on the Measured Vertical Rail Displacement  
The vertical rail displacement measurements in the test sections were used to analyze the track impact effects for a train passing over the rail joint gaps and very sharply curved CWRs. The TIFs calculated from the displacement response and the dynamic wheel fluctuation were then compared. The track impact effect can be generated simultaneously by the load and displacement, and the typical TIF can be calculated using the dynamic wheel load fluctuation. In this study, however, the effects of wheel load on the vertical rail displacement of the rail joint (amplitude of the displacement response) and the resulting track impact were experimentally estimated. These values were then compared with the typical TIF based on the wheel-rail contact load (dynamic wheel load fluctuation). On the basis of the displacement measurement results, the TIFs were calculated and compared. In this study, the dynamic rail displacement measurement results for track subjected to a passing train were replaced with pseudo-static displacement values using lowpass filtering [2]. The TIFs based on displacement were calculated according to the ratio of dynamic vertical rail displacement (\(D_{dyn}\)) to pseudo-static vertical rail displacement (\(D_{sta}\)) using Equation (3). The results of this conversion are shown in Figure 6.  
\[\text{Track impact factor (TIF)}\;i=D_{dyn}/D_{sta} \tag{3}\]  
Figure 7a shows the TIF based on the vertical rail displacement measured at the rail joints after the first wheel has passed; the vertical rail displacement decreased sharply after the second wheel passes, and then increased rapidly to the maximum value. This is because of the difference in rail displacement generated when the wheel passes through the two adjacent rails of the joint, resulting in a sharp displacement difference at the ends of the rail joint and an increase in displacement amplitude. The experiments suggested that this may damage the fishplates of the rail joint and affect the end rails. However, as shown in Figure 7b,c, the displacement response characteristics of the rail joint in Figure 7a did not occur in the CWR, thereby confirming the continuity of the time-displacement response. Table 4 compares the TIFs calculated using the dynamic wheel load fluctuation derived from the corresponding measurements and those calculated using the vertical rail displacement measurements. As shown in Table 4, on the basis of section B, which showed the lowest impact factor, this study compared the rail joint (section A) with section C, which used the same CWR. According to the comparison of TIF evaluation methods in Table 4, the TIFs of the rail joints (Section A) are higher than those of the CWRs in terms of both wheel load fluctuation and displacement response by 57.33% and 24.16%, respectively. This is because the load and displacement are relatively high owing to the rail joint gap and vertical rail displacement difference. Figure 7: Comparison of the measured vertical rail displacement between dynamic and pseudo-static for each test section: (**a**) section A; (**b**) section B; and (**c**) section C.  
This study experimentally determined that rail joints amplified the wheel-rail contact load (dynamic wheel load and its fluctuation) and increased the impact load to the track and wheel. The level of impact based on the displacement response according to increased dynamic vertical rail displacement caused by gaps in the rail joints was also higher than that for the CWRs, despite the higher train speed. The TIF evaluated using the dynamic wheel load fluctuation can directly affect the wheel and rail by increasing the damage and the impact load to the track components. The increased TIF owing to increased displacement is expected to cause damage to the fishplates of the rail joints and the rail seats of the rail joint sleepers.