## 2 Optical Inspection Technologies in Railways
### Monitoring Subjects  
Monitoring in the railway sector involves many different objects and conditions. In the following section, we explain and clarify various monitoring subjects that are detected by optical sensors.  
A railway track consists of rails, fasteners, sleepers and ballast as well as the underlying subsoil (see Figure 1). It constantly degrades with use. Sleepers are an important part of the track system. They are transverse elements that are used for the rigidity and fastening of the rails in the corresponding track gauge [14]. In combination with the fastening system, they absorb the dynamic train loads and distribute the load to the ballast [15]. They can be made of different materials such as concrete, wood, or steel. In the following section, we will concentrate on concrete sleepers, as these are most often used. Vegetation on railway tracks is special problem. The vegetation changes the elasticity of the track bed. The ballast fulfills the task of absorbing load peaks that act on the track due to traffic [16]. A change therefore represents a risk.  
In electric rail systems, tramways, and overhead buses, the multiple units, locomotives or buses pick up the electric power via a pantograph in the Overhead Contact System (OCS) (see Figure 2). The OCS consists of many individual components, including the overhead contact line, reinforcement and switch cross conductors, longitudinal and transverse support structures, voltage protection units, and insulators. Overhead Contact Lines (OCL) are given a horizontal zigzag pattern to provide a uniform contact with the pantograph. The pantograph causes repetitive contact along the wires, which wears them out. Defects or failures in this system lead to delays and possibly to safety risks. It is therefore important to guarantee ideal contact between the pantograph and the overhead line [17]. The development of pantographs worldwide can be seen as an evolutionary process: from two-arm pantographs with a complex structure and high weight to single-arm pantographs with a simple structure, less weight, and more flexibility [17]. When monitoring the contact wire, it is not only the position that is of interest but also the degree of wear. Its main function is to transmit power through the sliding contact. The main causes of failure are  
Figure 1: Components of a railway track.  
poor positioning and heavy wear [18]. Improper positioning or sagging of the contact wire can cause the pantograph to become detached from the contact wire, which, in turn, can lead to destruction of the OCL. This can be caused by weather conditions or incorrect installation [18]. The wear of the contact wire is determined on the assumption that the contact wire is round and the diameter of the unused contact wire is known. The remaining thickness can therefore be determined by calculating the measured diameter [11].  
Tunnels are an important infrastructure facility. Through mountains, under water, and bypassing obstacles, they provide time-efficient connections for passenger and freight traffic (Figure 3 left). Tunnels are constantly stressed by the environment and human uses. Water ingress and structural deformations have a significant impact on safety. Inspections must be carried out regularly to evaluate the condition of the structure. The purpose of recording the clearance is to record the free area above and next to the tracks in order to carry out a collision test with objects such as vegetation (Figure 3 right).