# Study of Rail Squat Characteristics through Analysis of Train Axle Box Acceleration Frequency
## 1 Introduction  
Early detection of abnormal railway behavior and rapid performance of maintenance work can prevent traffic blocks and minimize long-term costs of railway infrastructure. The present study was conducted by focusing on the detection of squats, which are small defects that start on the rail surface [1, 2, 3]. Squats begin as a small groove, corrugation, or weld portion [4, 5]. When squats are detected in their early stage and the deterioration is mild, maintenance may be easily performed by polishing a thin layer of the rail surface. Since severe squats may require to replacement of the rail, early detection of squats may significantly reduce rail maintenance cost. The total railway length in Korea (excluding urban railways) is about 2750 km, including 3400 bridges, 840 tunnels, and 1241 stations. This system transports over 420,000 passengers via 2000 trains each day. The cost of replacing a rail is high, and train delay or derailment due to defects of rails can give the public a negative impression of the railway system. Therefore, it is critical to prevent disruptions of system operation. Keeping rail in good condition requires systematic and periodic testing. A computer-based tool is used for such tests, and rail managers implement systematic maintenance to minimize the total cost and secure the long-term quality of the rail [6]. Various methods can be used to monitor rail track conditions [7, 8, 9, 10, 11]. Visual inspection and ultrasonic and eddy current measurement methods are used in Korea and many other countries to detect short-wavelength defects of railways [12, 13]. However, these types of tests are effective only in cases in which the rail performance is decreased due to severe damage; they are not optimal methods for maintenance. In addition, visual inspection requires a large amount of labor, and different results may be derived by different workers [14]. Therefore, it is necessary to develop a method to automatically detect irregular defects such as squats, and a monitoring system to efficiently evaluate the conditions of tracks, including rails. In the Netherlands, axle box acceleration (ABA) has been used since the mid-1980s to detect defects such as rail squats and poor welding [15]. Compared to other methods, ABA costs less and is easy to maintain. Recently, accelerometer-based methods have been proposed to monitor rail conditions [16; 17; 18; 19]. [PERSON] et al. [20] suggested a mixed filtering (Kalman filter and band-pass filter) method for estimating the irregularity of rails by using accelerometers mounted on wheel axles and bogies. [PERSON] et al. [21] presented a communication protocol between accelerometers installed in a train system to monitor the typical dynamic behavior of a train. The ABA method has been used to detect rail defects, including corrugations and welds of rails as well as of insulation joints (seams) [22; 23; 24]. However, the ABA-based measurement method is not capable of detecting all types of squats. Extensive detection is impossible because squats are usually generated randomly and only one squat is generated at a position. In addition, the ABA signals (especially light rail squats) may not be easily detected and analyzed without appropriate equipment and signal processing. This paper presents squat detection results obtained from the system applied in the present study, which was realized and verified through on-site tests on general railways in Korea. The system developed in the present study can be mounted on trains in operation to perform continuous monitoring of the entire railway infrastructure at a relatively low cost through continuous updates. In addition, the developed system is suitable for the task of addressing dramatic performance decreases, after which appropriate maintenance may not be applied due to the long cycle of general tests. The system developed in the present study may be applied to various commercial trains to significantly improve the safety of railway operation and reduce the life-cycle cost of the entire infrastructure.