# Rail Diagnostics Based on Ultrasonic Guided Waves: An Overview
## 3 Systems for Rail Defect Detection Based on Ultrasonic Guided Waves: Classification  
Substantial research efforts have been devoted to the study of systems exploiting ultrasonic guided waves for the detection of rail defects. More specifically, the following two broad categories of systems have been investigated:  
1. _On-board systems_[40, 45, 46, 47, 48, 49]-The measurement equipment is embarked on board a moving vehicle (e.g., an inspection cart), allowing for scanning the head of the rails on which it is riding. The detection of defects in the scanned rails is achieved through a pitch-catch mechanism. Initial implementations were based on an active approach, in which a source of ultrasounds was used to inject UGW into the rails, and sensing was accomplished through an array of transducers. More recent implementations adopt a passive approach, in which the ultrasonic excitation is generated by the rolling wheels of the moving vehicle. The approaches based on on-board systems have been promoted, among others, by the _Experimental Mechanics & NDE Laboratory of the University of California_, San Diego, USA.
2. _Land-based systems_[9, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67]-The employed equipment is placed on the ground near to the railway infrastructure, and its actuators and sensors are attached to the rails under test. The detection of defects in the rails is achieved through a pulse-echo approach. A lot of contributions about this topic have been published by the _Sensor Science and Technology Department_ of the _Council for Scientific and Industrial Research_, South Africa. The _ultrasonic broken rail detection_ (UBRD) system has been developed by this institution and put in use on the Oreline in South Africa to detect the presence of a fully broken rail; the system is evolved to implement an early defect detection system. In recent years, important results in this research field have been achieved by two Chinese research groups. In fact, significant contributions about the best methods to excite and detect propagation modes in a rail, and about feasible methods for rail defect location have been published by various researchers working at the Beijing Jiaotong University (_School of Mechanical, Electronic, and Control Engineering_, and _Key Laboratory of Vehicle Advanced Manufacturing, Measuring and Control Technology_). Moreover, an electronic system able to efficiently excite ultrasonic guided waves in rails has been developed by various researchers working at the Xi'an University of Technology (_Department of Electronic Engineering_).  
In the remaining part of this manuscript, both categories of systems are taken into consideration. In particular, the architecture of these systems is described in Section 4. Sections 5-7 are devoted to the implementation of on-board approaches, to that of the UBRD system, and to the evolution of the UBRD system towards early defect detection and monitoring. In Section 8, commercial projects like RailAcoustic (by Enekom), or university studies, like those accomplished by the Beijing Jiaotong University or the Xi'an University of Technology are summarized. Finally, various results about the performance provided by each of the considered systems are illustrated in Section 9.