# Measurement Repeatability of Rail Wheel Loads Caused by Rolling Surface Damages
## 2 Analysis of Rolling-Stock Wheel-Rolling Surface Damage  
During rolling stock inspections at railway stations, wheel damage is detected by templates and other devices (Figure 1). An external inspection identifies visible damages, such as wheel flats or cracks, and templates are used to inspect the wheel profile. Ultrasonic and magnetic defectoscopes are used to detect cracks in the wheel. The main damages of the rail wheels are the wear of the rolling surface, thinning of the flange, flats, cracks, and metal folds of the surface. The main types (shapes) of wheel-rolling surface damages are shown in Figure 2.  
Flange damage. The wheel flange prevents the wheel from derailing. A wheel is considered damaged when its flange is critically thinned (up to 25 mm). Metal folds. Metal folds occur in the presence of thermomechanical damage [24]. Intense plastic deformation of the metal occurs due to sudden braking, short-term wheel slip, wheel jumping, or sudden heating of the wheel metal, and then sudden cooling [25]. Several metal folds can be formed on the rolling surface of the wheel, as well as being in one or several layers. When the wheel of a passenger car has a metal fold up to 0.5 mm, or a freight wagon up to 1 mm, operation of the wagon is prohibited. Rolling surface wear. Most publications on the long-term interaction between rolling stock and track deal with the wear of rail and wheel [26; 27; 28]. The phenomenon of wheel wear has been extensively studied [29]. With the wear of the rolling surface of the wheel, train resistance to movement and the wheel/rail adhesion increase [30]. The wear of the rolling surface of the wheel is divided into even and uneven. Figure 1: Wheel measurement template and measurement with a ruler. Figure 2: Main nature of damage of wheel rolling surface. Uneven wear. This wear is special in that it does not have a dimensional extremity (such as the deepest point), so sometimes this type of wear is difficult to detect by the values of the dynamic diagnostic indicators [31]. The consequence of such wear is isolated wheel irregularities [32]. Flats. These are the most common wheel damage caused by wheel slip or a brake pad stuck [33], as shown in Figure 3.  
Wheel flats also occur from wheel slippage, wheel jamming, or braking equipment failures. Flats occur much more often in winter than in summer. The main reasons for the formation of the wheel flat are shown in Figure 4.  
Dynamic measurement methods include the vibration acceleration method, machine vision method, and YOLO deep learning network methods for wheel-rolling surface damage discovery. The operation of automatic crack detection system implemented based on YOLOv2 target detection, which improves the detection accuracy, is analyzed in the study [34]. Chinese researchers developed [35] a YOLOv3-based railway track line detection method that improved detection accuracy and speed. Furthermore, it preferred to incorporate lasers with images and, therefore, resulted in an structured light vision sensor (SLVS) sensor for assessing localized faults on wheel, such as a crack or wheel flat. Various diagnostic systems can determine the magnitude of the damage based on the impact force. There are a lot of methods in the scientific literature to determine the correlation between these values [12] and the principles by which these systems operate [36]. On the other hand, rolling-stock wheelsets oscillate when rolling on a rail track [37] (see Figure 5), so the wheel-rail contact points are not always on the same circle of the wheel rolling surface [38]. Different detection methods have their advantages and disadvantages. Some methods have low detection accuracy or high actual operating costs. Therefore, the search for a more precise method has become a relevant issue. Figure 4: Main reasons of wheel flat formation. Figure 3: Sample of wagon wheel damage—flat. Due to oscillation processes, the damaged wheel impacts on the rail at different surface points at each time, and the impact force varies.