# Optical Fibres for Condition Monitoring of Railway Infrastructure--Encouraging Data Source or
## 3 Materials and Methods  
For the study, two DAS-based systems were installed in the telecommunication room in Muensingen, Switzerland (Figure 2). Interrogation unit 1 (IU1) runs along the Muensingen-Bern route and recorded events there on a total of 1440 channels. The route section between Muensingen-Thun-Spiez was monitored with the IU2 on a total of 2724 channels. Considering a 10 m spatial resolution, the total length of the optical fibre between Bern and Spiez was 41.640 km, while the track length from Bern to Spiez was 39.080 km. The discrepancy of 2.560 km was due to both the cable routing and track layout. Firstly, an optical fibre is not always routed parallel to the track and does not always cross the track or, for example, lead into railway station buildings and subsequently run parallel to the track again. Secondly, fibre optic cables are wound at specific intervals in order to have reserves in the event of fibre defects. The system utilises C-OTDR and processes the backscattered signal as already discussed. In theory, the interrogation unit (IU) is capable of monitoring linear assets with an extension of up to 50 km. The minimum sampling rate of the system is 1 kilohertz (kHz) and can be gradually increased up to 20 kHz, though, it was 2.5 kHz (2500 samples per second) in the presented study. The same applies to the spatial resolution; starting with a relatively rough resolution of 20 m, the system is able to operate with a resolution of 7.5 m. This corresponds to a segment length or \"channel\" of 7.5 m. Each channel or segment length corresponds to a spatial resolution. A spatial resolution of 10 m, however, was chosen for this study. In conclusion, considering all the theoretical sensing capabilities of the system, a 1000 m optical fibre with a resolution of 10 m led to 100 channels. In addition to the resolution, the maximum number of channels that an IU can process is also limited. If the events were sampled at a rate of 2.5 kHz, then the IU will deliver for each time  
Figure 2: (**a**) Schematic track and fibre layout of the monitored route between Bern Wankdorf and Spiez, located in Switzerland; (**b**) rack mounted interrogator units located in the telecommunications room in Muensingen side-by-side with the processing unit at the top. sample and channel an averaged intensity over the chosen section length. This obviates the need for a complete replacement and reinstallation of the fibre in such a case (more details about the specific section length and optical fibre are displayed in Table 1). Optical fibres are generally deployed in bundles together in combination with other cables within cable conduits alongside railway tracks. The illustration in the figure above is sufficiently accurate for a general view for both the layout of the optical fibre and track as well as for a visualisation of the actual traffic. Knowledge about the exact fibre and track layout is decisive, however, when it comes to in-depth signal analyses. This is not only because both the track and cable layout vary, but is also due to divergence in the length of fibre and track. As already noted, the difference in length is more than 2.5 km, the over-length being on the fibre side. The points where excess lengths occur must be identified and subsequently taken into account in the analyses. DAS is a truly an intrinsically distributed measurement technology where the light beam never leaves the fibre and is divided into equidistant measurement points. The measurement takes place in the fibre itself, therefore the fibre length and the position of the fibre are substantial. Sections with an unclear fibre path must be identified and documented. Otherwise, this can lead to incorrect result interpretations and subsequently result in wrong conclusions regarding the applicability of the system for railway condition monitoring and asset management. The monitored track is an existing and mostly double line in Switzerland. Some track sections have very heavy loads with an average of nine trains per hour.