# Deep Learning (Fast R-CNN)-Based Evaluation of Rail Surface Defects
## 3 Rail Internal Defects Specificity, Characteristic  
### Overview  
Rail surface damage on the currently operational tracks may exhibit various magnitudes and types. Previous studies have mainly focused on investigating the impact of factors, such as the magnitude of wheel-rail contact load and stress variations due to contact area, on the formation of cracks or damages on the rail surface. An inspector faces various difficulties in visually detecting rail surface damage when inspecting from a long distance. Upon conducting an extensive examination of the affected area, it was determined that the rail surface damage may yield different results depending on the perspective of the inspector. This study analyzed rail internal defects occurring at various depths and lengths in such surface damages through SEM testing. The flowchart of the SEM test procedure is illustrated in Figure 3. The damaged rails were sampled through field investigation. An analysis was conducted to distinguish the types of damage based on the examination of the characteristics of cracks that extend from the rail surface into the interior of the rail. The surface defects on the rail surface, which occur in the direction of the train travel due to wheel-rail contact (rolling contact fatigue, RCF), are classified into two types: rolling direction (C) defects caused by the contact, and lateral wheel displacement direction (L) defects caused by the lateral movement of the wheels. In addition, an SEM test was conducted. The conical wheel shape of the train's wheel axle is crucial in terms of the contact with the rail. The turning radius of a vehicle may vary, and the rolling contact occurs in varying degrees when the wheel and rail surfaces are in contact. This study investigated the phenomenon of non-uniform crack patterns resulting from wheel-rail contact in the field. The damage characteristics of patterns with different crack orientations were investigated through SEM testing. In the field measurement of this study, rail samples (7-15 m) with damaged rail surfaces were collected from an operational urban railway service. A total of 100 specimens were cut at 30 cm intervals. In the SEM testing, damaged rail specimens were processed to demonstrate the correlation between surface defects and internal defects (cracks). Surface defect-specific specimens were processed and mounted, and they were then polished to measure the depth, length, and angle of the cracks. The crack angle indicates the rate of growth of a crack that progresses in a certain direction of length [6; 7; 8]. In this study, the cross-sectional observation was performed on each sample by cutting it in the appropriate direction and mounting the cross-section. After mounting, polishing was carried out using 1 \(\upmu\)m diamond suspension. The crack length, depth, and angle were measured using an electron scanning microscope (JEOL JSM-IT500 with OXFORD ULTIM MAX). Figure 4 shows an examination of rail tissue and equipment test using scanning electron microscopy (SEM). There are some challenges in the measurement of rail-internal cracks using non-destructive testing. The level of rail surface damage was quantified, and the correlation between surface and internal defects was accordingly examined. The processed sample shown in Figure 4a was observed via SEM using the setup shown in Figure 4b. To demonstrate the correlation between surface defects and internal defects (cracks) in aged rail specimens, damaged rail observations were conducted by processing the rail samples, as shown in Figure 4c,d, The damaged rail is presented in Figure 4c. For cutting, a mechanical cutting method was applied to process the sample to 20 mm \(\times\) 20 mm or less in the area containing the crack. To grind or polish the cut specimen, as shown in Figure 4d, it was made into a certain shape using a resin; this process is called mounting.