## 4 Discussion
The advantage of passive grinding is high speed and high efficiency, which can complete rail maintenance in a short \"skylight time\". A low removal rate will make passive grinding lose its superiority. The most convenient solution to improve grinding efficiency is to increase the grinding pressure load. The results demonstrate a significant increase in grinding efficiency from 13.4 mg/s to 107.5 mg/s with the increase in grinding load from 120 N to 320 N, as shown in Figure 4.  
The removal principle of passive grinding can be analyzed by analogy with heavy load fixed force grinding. The relationship between grinding removal rate and grinding pressure load is obtained through the comparative analysis of cutting models and expressed as Equation (1) [26]. \[Q_{w}=(\frac{Q_{s}}{Q_{s}+q_{s}})K{F_{n}}{v_{s}} \tag{1}\]  
In formula (1), the parameter \(K\) is related to the grinding force characteristics of passive grinding, while \(q_{s}\) is related to the quality of the abrasive particles themselves. The value of \(q_{s}\) depends on the chemical composition of the abrasive particles rather than the size or shape. High zirconium content abrasive is commonly used in rail grinding. The high zirconium content of zirconium corundum abrasive has a lower \(q_{s}\) value in grinding steel because it has a higher melting point and chemical stability than the alumina abrasive [26]. It can be viewed in the formula that under ideal conditions if the abrasive particles never wear (\(q_{s}=0\)), the rail removal rate is only related to the grinding pressure under a certain speed. The higher the grinding pressure, the higher the grinding removal rate. However, the abrasive must be worn in actual grinding. If \(q_{s}\) is set as a constant value, the greater the wear rate of the grinding wheel (\(Q_{s}\)), the higher the rail removal rate (\(Q_{w}\)). These demonstrate that the increase of passive grinding efficiency is related to the pressure load and related to the grinding wheel wear. Although more grinding wheel wear can improve grinding, the service life of grinding wheels plays a non-negligible role in the economics of grinding. The grinding ratio drops off with the increase of grinding pressure load, which shows the durability of grinding wheel decreases as the working pressure load increases. The phenomenon of frequency wheel replacement caused by excessive grinding load is undesirable to see. When observing the surface morphology of grinding wheel after grinding test, there is no obvious wear on the grinding surface under the load below 200 N. However, when the grinding load exceeds 240 N, visible fissures would appear on the grinding surface of the wheel, and the abrasive will break or even fall off to form holes, as shown in Figure 9. If the grinding wheel shows serious abrasion only after 30 s grinding, it is not the normal self-sharpening of the grinding wheel but the damage caused by excessive pressure load. Therefore, it is not appropriate for the grinding pressure load to exceed 2400 N in actual working conditions. This is essential for the application of passive grinding in practice. Frequent replacement of grinding wheels is often caused by excessive grinding pressure load during online grinding, which adds significant economic costs. So, the influence of the increase of grinding pressure load on the passive grinding is not just a favorable relationship. The enhancement of grinding load will not only increase the grinding efficiency but reduce the service life of grinding wheel and the rail surface quality after grinding. It is inseparably linked to the variation of the grinding force. In passive grinding, the grinding pressure load is part of the necessary conditions to produce the grinding behavior, and it also has a very profound impact on the grinding force.