## 2 Materials and Methods  
### Self-Designed Passive Grinding Simulator  
A passive grinding simulator was self-designed based on the tool grinder as shown in Figure 1a, and its grinding schematic is shown in Figure 1b. The passive grinding process of contact mode is shown in Figure 1c, and the combination of rail sample is shown in Figure 1d. The test device is primarily composed of the part of round rail sample and the support part of the passive grinding wheel. The main characteristic of the test equipment is that the grinding wheel (4) had no active rotating power, while, the round rail specimen (3) rotated actively driven by the spindle of the motor (2) providing the rotating power. The grinding wheel (4) was fixed on one side of the L-form locating plate (6) at a certain angle through the bearing bar and bearing bracket (5), which could rotate freely without resistance under the action of external forces. The grinding angle of the grinding wheel (4) on the rail specimen (3) was fixed at 45\({}^{\circ}\). In rail grinding experiment, the rail specimen (3) rotated at a certain linear speed to simulate the speed of the grinding train. The air compressor and storage tank (11) were used to provide pressure load by driving the sliding based (14) to move through the pneumatic cylinder (12) to make the grinding wheel (4) contact with the round rail specimen (3) and apply a pressure load. In this way, the relative motion between them was produced and the grinding was carried out.  
In order to test the grinding temperature of the passive grinding area in the test, a temperature measuring system was designed, and its structure is shown in Figure 2. Figure 2a shows the side sectional view of the rail sample, and Figure 2b shows the bottom substrate diagram. The NiCr-NiSi thermocouple (IV) wrapped with insulating film clamped between the rail specimens for insulation and was fixed with mica fixture (II). Since the rail specimen (I) was rotating during the test, two conductive copper rings (VII) fixed on the insulation ring (VI) were embedded on the top of the bottom substrate. The copper brush (VIII) was always in contact with the conductive ring (VII) so that the electrical signal generated by the thermocouple (IV) could be continuously received while grinding. In addition, an infrared thermometer (X) was installed externally during grinding to assist in recording the grinding temperature for verification purposes.  
During the grinding test process, the main control cabinet of the test machine (1) controlled the rail rotation speed, contact pressure and grinding time. The test machine chassis base (13) was used to support the whole grinding mechanism and collect grinding debris. The three-dimensional mechanical sensor (8) was fixed between the L-form locating plate (6) and the sliding base (14), and the laser infrared thermometer (7) was fixed on the lateral side of the L-form locating plate (6). The test equipment could record mechanical and temperature signals by acquisition card and signal amplifier (9) during grinding, so that the grinding pressure, the grinding force along the direction of the grinding wheel and the grinding temperature at the grinding contact point could be recorded synchronously at the computer terminal (10). Since the grinding wheel and the supporting part of the grinding wheel had a dead weight, the tangential component of gravity direction was measured and zeroed before each test.