# Optical Fibres for Condition Monitoring of Railway Infrastructure--Encouraging Data Source or
## 1 Introduction
A method for increasing the measurement frequency to assess the condition of tracks provides sensors mounted on in-service vehicles/trains (onboard monitoring (OBM)), including axle box accelerometers [3; 4] or bogie movement measurements [5; 6; 7]. These data could be a valuable data source for critical assets and sections and have been examined for almost 20 years. Nevertheless, measurements on a broad scale are still not available and many measurement campaigns struggle to deliver reliable data, resulting in them not yet being proven. These systems are assigned to the first group \"vehicle measures infrastructure\". The latter discussed systems aim not only to deliver data on the track condition, but also to assess the health status of vehicle components. Such systems can be classified into the second group, \"vehicle measures vehicle\". In recent years, the application of sensors on turnouts [8] and bridges [9] has been thoroughly investigated. These approaches are very useful to gain knowledge about the condition and behaviour of these assets but deliver only information about specific turnouts or bridges at one location. Furthermore, gaining knowledge on the health status of these assets is becoming more and more decisive as experienced experts--know-how owners with long-term manual inspection experience--retire from service and their valuable knowledge is lost with them. Manual inspections are highly subjective procedures, they are not to be regarded as reproducible and are naturally time-consuming and costly. Some of the aspects of this call to equip thousands of turnouts and bridges with sensors and measures on this scale are scarcely affordable for any IM. Sensors mounted on assets of interest are assigned to the third group, \"infrastructure measures infrastructure\". Lastly, another noteworthy data source that IMs rely on is wayside train monitoring systems (WTMS): \"infrastructure measures vehicle\". The main field of application is, accordingly, the monitoring and assessment of rolling stock conditions. WTMS have been gradually expanded and provide additional information about the infrastructure-damaging conditions of vehicles such as dragging equipment or whether vehicles comply with the clearance profile [10,11]. Trackside sensors for rolling stock observations have been used for a long time, are proven systems and monitor the condition of each train passing at the location where they are deployed. Nonetheless, these are local measurements at specific points installed at neuralgic points within the network. The effective extent of the measurement system is determined by the quantity of equipment in use and inevitable gaps can lead to some undetected failures and incidents. All the above-noted systems have strengths and shortcomings. However, an IM's strategy and budgetary situation determine the number of installed WTMS. On the one hand, wayside condition monitoring sensors are very costly and hence it is not possible to install them every few kilometres. On the other hand, each sensor equipped for tracking requirements must be maintained from time to time and, in cases of track maintenance, they may have to be removed to prevent sensor damage. The removal of such equipment is not only time-consuming, it also influences the maintained section length in a specific track closure. Permanent measurements, such as axle counters, provide extensive information about vehicle performance at any point in time for each train crossing, but only do so at fixed positions, where measuring equipment of this kind is installed. In contrast, continuous measurements such as track recording cars deliver information at one point in time but do so only for a specific longitude. As a result, there is a lack of data, measurement and information about the track condition. Based on the evaluation methods discussed, the question arises whether there are further data sources without the requirement of additional sensor installations and track closures for measurements.