# On the Fault Detection and Diagnosis of Railway Switch and Crossing Systems: An Overview
Therefore, recently, both railway infrastructure managers and researchers have started focusing on developing reliable CM techniques for S&C systems [15]. This paper thus first provides the fundamentals of S&C systems by focusing on their mechanisms, the sensors and controllers installed, the actuators used, and their locking systems. Then, the different failure modes of railway S&C systems are briefly discussed. Finally, this paper provides a comprehensive overview of the existing CM techniques for dealing with the fault detection and diagnosis (FDD) and fault prognosis (sometimes) of railway S&C systems. In an attempt to better review the existing FDD methods for S&C systems, first these methods are classified into two groups, model-based approaches and data-driven approaches, and then three categories are considered for each group: (1) fault detection (FD) methods, which deal only with detecting the fault; (2) FDD methods that detect the presence or not of the fault and then diagnose its location and roots; and finally (3), fault prognosis (FP) methods in which remaining useful  
Figure 1: World market regions with market volume (EUR) and market development until 2022 (%) by region [3]. life (RUL) prediction is performed. The approaches described in this paper are structured as shown in Figure 2.  
This paper is organized as follows: Section 2 introduces the fundamentals (mechanisms, sensors and controllers, actuators, and locking systems) of S&C systems, followed by a brief description of the different S&C failure modes and their causes. The different existing FDD algorithms applied to S&C systems are detailed in Section 3. Finally, a conclusion is given in Section 4. To the authors' knowledge, there is no review paper that focuses on reviewing the existing FDD techniques for the CM of railway S&C systems except the one published by [PERSON] et al. [16] a decade ago. To avoid any duplication, this paper will not review the papers already considered in the aforementioned review paper. It will just focus on giving an overview of the recent published papers related to the CM of railway S&C systems. In addition, the FDD methods applied for S&C systems were classified into non-dynamic and dynamic algorithms in the previous review paper, whereas in this paper, the FDD methods developed/applied to railway S&C systems are classified into model-based and data-driven methods. Another difference of this review paper is that it considers all research papers (that deal with detection and diagnosis (and sometimes prognosis) of different S&C system faults) from all around the world, and is not only narrowed to those within UK territory, contrary to the review paper of [PERSON] et al. [16]. Fundamentals of S&C Systems: Actuation Mechanisms, Sensors and Controllers, Locking Systems, and Their Failure Modes  
A traditional railway S&C system consists of several individual elements such as the switch layout, actuation mechanism, sensor and control system, and detection and locking mechanism. In the following sections, these elements of a conventional switch system are described and new advances in each element are discussed in brief. Most conventional railway S&C systems use a switch layout which consists of two switch rails and two stock rails [17]. Figure 3 shows a conventional switch layout used in a railway network, which can direct the rail vehicle to two different routes, normal and reverse. The switch rail in its closed position is locked adjacent to its stock rail, and the other switch rail is locked in the open position to allow the wheel to pass over the rail on the open side. The stretcher bars are connected between two switch rails to maintain free wheel passage. Point operating equipment (POE) is the actuator that moves the railway switch and ensures that the closed switch rail is always placed, locked, and detected relative to its associated stock rail [18].