# Review on Rail Damage Detection Technologies for High-Speed Trains
## 3 Non-Destructive Testing Methods for High-Speed Train Rails  
### Ultrasonic Waveguide Testing Methods  
The detection of surface defects on railway tracks based on ultrasonic waveguide technology is currently widely used in the field of high-speed train rail surface defect detection. With the development of intelligent transportation technology, the requirements for the service life and quality of railway tracks are becoming increasingly high. At the same time, various types of surface quality defects, such as dents, cracks, surface burns, weld defects, joint defects, etc., have also occurred during the service of railway tracks. The most important components in the ultrasonic waveguide detection process are the linear ultrasonic head and the angular ultrasonic head [28]. At the same time, different angles are arranged according to the layout of the railway track, with the aim of covering relevant areas in different regions of the railway track. In Germany, two linear ultrasonic heads emitting longitudinal waves are used to simultaneously detect contact surfaces running parallel to the railway tracks, and a matching reflector is used to detect the height of the railway tracks. One of the two ultrasonic heads adopts a dual element setup, installed on the rail head and reflector near the contact surface of the rail, while the other ultrasonic head is installed on the rail web and rail foot. Normally, the installation covers an angle range from +35 \({}^{\circ}\) to +70 \({}^{\circ}\) and from -35 \({}^{\circ}\) to -70 \({}^{\circ}\), as shown below in Figure 6.  
By using ultrasonic sensors to collect the characteristics of rail defects, the quality and accuracy of the surface defect feature map of the rail are relatively high, and the acquisition  
Figure 6: Schematic diagram of ultrasonic head installation. efficiency is high. However, in order to obtain complete and high-quality rail defect feature images [29], the ultrasonic head still needs to be installed at different angles. Shown in Figure 7 is a schematic diagram of ultrasonic wave acquisition at different angles. Electromagnetic ultrasonic testing technology excites and receives ultrasonic waves through the principle of electromagnetic coupling. A coil with alternating current is placed near the surface of the rail to be tested. Under the influence of the alternating current, eddy currents are generated inside the rail. The eddy currents inside the rail are affected by the Lorentz force under the action of the alternating magnetic field, generating ultrasonic waves inside the rail. Compared with traditional ultrasonic testing techniques, electromagnetic ultrasound has the characteristics of non-contact implementation, no need for coupling agents, and fast detection speed [30; 31]. Currently, [PERSON] et al. in the United States have used electromagnetic ultrasound testing technology to achieve damage detection in steel rails [32]. [PERSON] et al. [33] in China studied a digital signal processing method suitable for the electromagnetic ultrasonic testing of rail treads. This method can improve the signal-to-noise ratio of electromagnetic ultrasonic testing signals and achieve the localization and imaging of rail tread damage. Compared with the disadvantage of traditional ultrasonic detection technology being susceptible to external interference, laser ultrasonic detection technology can adapt to complex and harsh detection environments with high detection accuracy [34]. [PERSON] [35] et al. developed a non-contact air coupled laser ultrasonic detection system, demonstrating the applicability of laser ultrasonic programs in improving the ultrasonic detection performance of train axles, and it has been applied to the inspection of high-speed train axles by Italian railway companies. [PERSON] et al. [36] applied a laser electromagnetic ultrasound technology to defect detection in the rail waist area which can detect through-hole defects of 6 mm-8 mm in the rail waist area.