# Review on Rail Damage Detection Technologies for High-Speed Trains
## 3 Non-Destructive Testing Methods for High-Speed Train Rails
### Terahertz Detection Methods  
Terahertz (THz) radiation typically refers to electromagnetic waves with frequencies between 0.1 THz and 10 THz (wavelengths between 30 \(\upmu\)m and 3 mm), which fall between the microwave and infrared wavelengths and belong to the far-infrared band. For the online real-time detection of surface defects on high-speed train tracks, terahertz time-domain spectroscopy systems can achieve the detection of quality defects on the track surface. They mainly include a terahertz time-domain spectroscopy system based on the femtosecond laser excitation of photoconductive antennas. The terahertz optical path system is composed of a fiber-coupled terahertz transmitter, a fiber-coupled terahertz receiver, a 100 V high-voltage generator, a fiber-coupled delay line, a high-speed data acquisition board, a portable computer, an external lens mounting bracket and a TPX lens, control software, and an acrylic protective plate. The overall optoelectronic scheme of the whole fiber terahertz time-domain spectroscopy system is shown below in Figure 8.  
When terahertz waves are incident at a certain angle and come into contact with the surface of the steel rail through the sample, they are completely reflected, resulting in the total reflection of terahertz waves by the steel rail. The thickness detection of THz TDS using the reflective THz method involves the incident terahertz wave E (w) on a rail damage feature with a thickness of \(d\). Eper is emitted from the upper surface of the rail damage feature, with an arrival time denoted by T. E is emitted from below the sample, with an arrival time denoted by T. The thickness of the sample can be expressed as [45; 46]  
\[d=\frac{c\Delta T}{2\sqrt{n^{2}-\text{sin}\theta}}=\frac{c(T_{2}-T_{1})}{2\sqrt{ n^{2}-\text{sin}\theta}} \tag{4}\]  
Among them, \(c\) is the speed of light waves, and \(n\) is the refractive index of the material.  
When terahertz waves are incident at a certain angle, the THz waves reflected by the sample cannot be completely received by the terahertz receiver. To avoid this situation and ensure that terahertz waves are vertically incident on the sample, the formula can be simplified as  
\[d=\frac{c\Delta T}{2n}=\frac{c(T_{2}-T_{1})}{2n}\]  
For the internal defects of laminated steel rails, non-destructive testing is carried out using a terahertz time-domain system. The only improvements made are in signal and image processing, and the results obtained are not yet perfect. Therefore, it is necessary to start with the hardware of the terahertz system to break the limitations of the terahertz system itself and improve the quality of signals and images. At present, terahertz systems are bulky and only remain in laboratories, making it difficult to perform on-site detection. Under ideal conditions (such as a clean rail surface and a defect size \(\geq\) 0.1 mm), the detection rate of surface open cracks using terahertz time-domain spectroscopy (THz TDS) or terahertz imaging technology can reach over 90%.  
Therefore, it is necessary to further optimize and improve various devices and processes to truly achieve the miniaturization of terahertz systems and meet the requirements of on-site detection. With the development of deep learning, algorithms for detecting and classifying defects also need to be developed accordingly; otherwise, sometimes, defects may be missed or incorrectly detected due to improper algorithms.