## 3 Results
### Morphology and Roughness of Rail Surface Grinding Zone  
The surface quality of rails is one of the most important factors in evaluating rail maintenance, in which the morphology and roughness of rail surface are the most direct manifestations of rail surface quality to affect the safety and stability of train operation. The optical morphologies of rail surface before and after passive grinding are shown in Figure 5. Before grinding, the rail surface was processed into the same surface topography and exhibited regular groove topographies as shown in Figure 5a. After grinding, the grooves were removed and replaced by fine grinding marks. It is found that grinding pressure has some effect on grinding morphology. As the grinding pressure load was 120 N, some coarse grooves that existed before grinding could not be completely removed, and there were some scattered shallow grinding marks on the rail surface, as shown in Figure 5b. When the grinding pressure load increased to 160 N, the grinding marks filled on the rail contact surface, while the grinding marks distributed uniformly. Less shallow grinding marks regions and common grinding marks regions could be found on grinding surface as shown in Figure 5c. With the increase of load to 200 N, the obvious plastic flow and flat could be observed as shown in Figure 5d. The plastic flow areas are caused by the accumulation of steel on both sides of the grinding mark by abrasive plow rail, while the abrasive leaves flat areas between the grinding marks after cutting the rail with the discharge of rail debris [29]. Moreover, when grinding pressure was further enhanced, the grinding marks were coarsened and some wear regions appeared on the rail surface as shown in Figure 5e-g. Combined with the grinding test, it can be found that due to the rotation of the grinding wheel with the movement of the rail and the deflection angle of the grinding wheel, the grinding action of grinding wheels on the rail is greatly affected by pressure during the grinding process. Insufficient pressure under low grinding pressure load will lead to grinding unevenness, resulting in a small area of shallow grinding marks. Excessive pressure load will generate heavy cutting of rail by grinding wheel, which may cause surface wear. The results illustrate that the increase of passive grinding pressure load will improve the grinding quality stability but over a certain limit will raise the possibility of wear on the rail surface, which is not conducive to grinding. The results can be confirmed by observation carried out on an optical 3D surface profilometer. Figure 6 shows the 3D surface topographies and profiles of the rail sample before grinding and after passive grinding under different loads. In Figure 6, the Sa and Sz are surface roughness and maximum topographic depth, respectively. Meanwhile, the grinding surface topographies of the rail are visually represented by the profile lines in the figures. According to the results shown in Figure 6, the surface roughness and topographic depth of the rails before and after grinding were quantitatively analyzed, and the results after repeated test and data statistics are shown in Figure 7. Based on Figures 6 and 7, the grinding pressure load could greatly affect the roughness and topographic depth of rail surface after passive grinding. Although the grooves are still present on the rail surface when the test grinding load is 120 N as shown in Figure 6b, the roughness and topographic depth of rail surface observably decrease from 2.613 \(\upmu\)m and 19.283 \(\upmu\)m to 0.459 \(\upmu\)m and 5.146 \(\upmu\)m as shown in Figure 7. With the grinding load raised to 160 N, the groove defects are completely removed from the rail surface as shown in Figure 6c, and the surface roughness and topographic depth further reduce to 0.210 \(\upmu\)m and 3.515 \(\upmu\)m as shown in Figure 7. While the grinding load is over 160 N, the elevation in the expression of surface roughness and topographic depth increases with the increase of grinding load.