## 4 Discussion  
Passive rail grinding is a new rail grinding strategy. Compared with the traditional active grinding method, it has the characteristics of speed and efficiency [12; 16], which can effectively make up for the deficiency of active grinding in railway preventive grinding [13; 17]. During the process of rail passive grinding, complex material removal and tribological behaviors take place between the grinding wheel and rail. However, it is difficult to detect and analyze passive grinding under the actual working condition, and  
Figure 14: XRD spectrum of debris under pressure load of 240 N.  
Figure 13: SEM images and EDS analysis of the grinding debris, (**a**) SEM of debris, (**b**) spherical melt debris, (**c**) serrated debris, (**d**) continuous banding debris. there is no test equipment available that is specialized in passive grinding. Therefore, a kind of small-scale simulation equipment of rail passive grinding was designed based on the tool grinding machine in this work. In practical applications, grinding pressure is a key factor in the effectiveness of passive grinding. Under different grinding pressure load conditions, not only the rail surface topographies after passive grinding are significantly diverse but also the service life of the grinding wheel is affected. In order to understand the passive grinding characteristics and develop process specifications for grinding pressure, a series of experiments were performed to study the influence of grinding pressure load on the rail material removal performance and the use of grinding wheels in passive grinding. According to experimental results, it can be found that the change of grinding pressure has a profound influence on the grinding effect. Concerning the grinding quality, the increase in grinding pressure load will lead to the change of rail surface topography and the increase of surface residual stress. In the experiment, as the grinding pressure load increases from 120 N to 320 N, the rail surface roughness first decreases and then increases, as shown in Figure 7. The first decline occurs because the surface defects are removed by passive grinding, which gives rise to the decrease of roughness and topographic depth. After that, the downward trend converts as the grinding pressure load increases sufficiently to completely remove the defects. The roughness and topographic depth increase with the increase of grinding pressure load. Finally, when the grinding pressure load increases to a certain level, the upward trend will not be prominent. It indicates that the surface roughness of the rail after passive grinding can be controlled within a certain range by adjusting the load, to meet the maintenance requirements of the rail. Moreover, under pressure load exceeding 280 N, the rail surface emerges to wear after grinding, which is harmful to the rail. In addition, the residual stress and plastic deformation on the rail surface increase with the grinding pressure load. These will shorten the service life of the rail because the residual stress and plastic deformation promote the formation and propagation of fatigue cracks [35]. However, there is not enough research to explain how residual stresses interact with grinding wheels or wheels in rolling contact. It is still an open point that is worth investigating. Therefore, after conversion, it is suggested that the actual passive grinding pressure load should be controlled between 1600 N and 2800 N according to the grinding quality. For maintenance efficiency, under low grinding pressure load, though passive grinding can be carried out, the defects may not be completely removed in the required time due to the low grinding efficiency. For instance, when the grinding load is 120 N, the grinding efficiency of a single test wheel on the rail is only 13.4 mg/s, as shown in Figure 4. After 30 s of passive grinding on the rail sample, the grooves existing on the rail surface are not completely removed, as shown in Figures 5b and 6b.