## 2 Materials and Methods
### Grinding Test  
The grinding test was employed on the self-designed passive grinding simulator. The parameters were designed according to the actual passive grinding conditions of the VOSSLOH \"HSG-2\" grinding train [14]. The speed of 80 km/h and the rotation angle of 45 deg are commonly used, and the depth of cut is around 0.05 mm per pass [12; 15]. Therefore, the rotational speed of the rail sample was designated as 2831 r/min to simulate the speed of the grinding train of 80 km/h. The purpose of this paper is to study the influence of grinding pressure on behavior of material removal from rail by grinding wheel. The setting of the grinding pressure depends on the pressure load, which is another most important parameter related to force loaded on grinding wheel and its contact area on rail. As shown in Figure 3, the contact arc length (_lc_) between grinding wheel and rail sample relates to the depth of cut (\(\alpha_{p}\)) and equivalent diameter of the grinding wheel (_d_), i.e., _lc_=(\(\alpha_{p}\cdot d\))\({}^{\frac{1}{2}}\)[26]. If the cutting geometric-contact length of the grinding wheel is approximately regarded as a straight line and the grinding depth is set to a uniform 0.05 mm, the contact area between the grinding wheel and rail can be calculated. According to the size of the test grinding wheel (80 \(\times\) 10 \(\times\) 10 mm) and actual grinding wheel (160 \(\times\) 74 \(\times\) 35 mm) for rail grinding, the test contact area is about 1/10 of the actual rail grinding; thus, the test load should be about 1/10 of the actual rail grinding, and then the same contact pressure load between the test and actual rail grinding can be ensured. In light of the pressure load of 2000 N used commonly in actual rail grinding [27; 28], six groups with different loading force were employed in this work and they were sequentially numbered as A to F. The tests were carried out under the same conditions of rail velocity of 22 m/s, wheel deflection angle of 45 deg and grinding time of 30 s. The grinding loads for groups A to F were set to 120 N, 160 N, 200 N, 240 N, 280 N and 320 N, which represent the actual grinding loads from 1200 N to 3200 N, respectively. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Elemental** & **C** & **Si** & **Mn** & **P** & **S** & **V** & **Nb** \\ \hline  & 0.65–0.76 & 0.15–0.58 & 0.70–1.20 & \(\leq\)0.035 & \(\leq\)0.030 & 0.030 & \(\leq\)0.010 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical compositions (wt%) of U71 Mn rail steel. \begin{table}
\begin{tabular}{c c} \hline \hline
**Grinding Wheel Type** & **Hot Pressed Resin Grinding Wheel** \\ \hline
**Abrasive** & fused alumina zirconia, brown fused alumina, ceramics \\  & corundum abrasives \\
**Abrasive size** & 14–60 mesh \\
**Binder** & high toughness heat-resistant resin \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Composition of passive grinding wheel. It should be noted that this simulation of passive grinding cannot completely recreate the process of rail high-speed grinding due to the limitation that the grinding wheel cannot work continuously on the straight rail. It results in the difference between the specific values of the test results and the practical results of high-speed rail grinding, such as the surface roughness of the rail after grinding. However, it does not affect the study of the influence of passive grinding parameters on the trend of grinding results and the grinding test of passive grinding wheels because this simulation focuses on passive grinding behavior. In addition, in order to make the grinding test results more visible, the test time of passive grinding was extended. In practical application, the grinding time of rail high-speed grinding is slightly less than 30 s.  
All experiments were conducted in the same room environment (temperature: 21 \({}^{\circ}\)C, humidity: 60%). Each group of experiments utilized new grinding wheels and was repeated 6 times to obtain the final experimental results.