**2. Preliminaries**

This section introduces the mechanical structure of a running gear system in a highspeed train. In this study, signal faults mainly caused by parts were selected as FD targets. Then, the research goals and problem statements of system design were discussed.

### *2.1. Introduction of a Running Gears System of a High-Speed Train*

The running gear system is an important system that affects the smooth running of high-speed trains. It improves the traction performance of high-speed trains and has the functions of generating power, buffering and supporting. The running gears system of high-speed trains include many complicated parts such as the axle box, traction drive, detection sensor, and spring device. Any malfunction from these parts may cause the carriage to shake during running and result in unpredictable consequences.

This paper aimed to analyze the running gears model and establish a data-driven FD model. The running gears system has such a multi-level complex structure. Therefore, it is difficult to build an FD model. As shown in Figure 1, many temperature sensors are arranged inside the gears. The test points of temperature sensors, for example, *A*1– *A*4 for axle box bearing temperature measuring point, *B*1–*B*3 for motor temperature measuring point, and *C*1– *C*4 for gear box temperature are shown in Figure 1 [31].

**Figure 1.** Structure of a running gears system.
