# Optical Fibres for Condition Monitoring of Railway Infrastructure--Encouraging Data Source or
## 3 Materials and Methods
Consequently, train headways of under two minutes are achieved in morning rush hours, leaving no room for operational disturbances. For these reasons it was not possible to deploy a new optical fibre cable and the existing glass fibre infrastructure installed along tracks was used in this project. Furthermore, no improvements were made regarding the bedding, routing and distance of the optical fibre to the track. The concrete duct protecting the cables inside is located predominantly on the left or right side, parallel to the track as shown in Figure 3a. This represents the ideal case, as the fibre runs parallel to the railway track and is at a constant distance from the track. Railway tracks are linear structures with both straight sections and sharp curves with very short radii and this may change the distance between the conduit as well as its routing. Furthermore, components such as rails, sleepers, ballast and turnouts are subject to wear and tear and must be maintained in order to obtain a certain track quality. Finally, at the end of their service life, they have to be replaced. For operational reasons, it may be necessary to install new turnouts, construct new stations or install new signalling systems. Each new installation needs power and must be linked to and integrated within existing system architectures. These circumstances may result in the need for new conduit and cable routing, affecting the layout of optical fibres. Thus, not only the superstructure changes. The illustration on the right in Figure 3b shows the track and fibre layout in which the fibre layout is not unambiguous. While one optical fibre runs on the left side of track 1, another one is deployed next to track 3 on the right. For any application or testing, DAS is adopted, and the fibre in question and its layout must be identified. It may also be the case that prior cable lines have not been dismantled or a cable even changes the trackside for various reasons, resulting in extra cable lengths. All these circumstances highlight why it is vital to identify the exact cable layout. To accomplish this, the route was followed at neuralgic points, such as stations, which are locations with spools and bridges resulting in a rough estimate of the fibre location. These points, also called calibration points, where the fibre makes a significant \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Section** & \begin{tabular}{c} **Number of** \\ **Stations** \\ \end{tabular} & \begin{tabular}{c} **Interrogator** \\ **Unit** \\ \end{tabular} & \
\begin{tabular}{c} **Section** \\ **Length** \\ \end{tabular} &
\begin{tabular}{c} **Fibre** \\ **Length** \\ \end{tabular} & **Coils** & **Delta** \\ \hline Bern & & & & & & \\ Wankdorf–Muensingen & 5 & IU1 & 14,262 m & 14,400 m & 1 & 138 m \\ Muensingen–Spiez & 7 & IU2 & 24,818 m & 27,240 m & 6 & 2422 m \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Section and optical fibre length. change in both the routing and position and length compared to track, were stationed, documented and referenced by GPS using longitude and latitude data. While for the track section between Bern and Muensingen, 19 calibration points were referenced; 32 calibration points were referenced for the Muensingen-Spiez section. The remaining points and fibre lengths were interpolated with a spatial resolution of 10 m. This process enables the identification of sections to be discarded because of bad signal quality and an unfavourable fibre layout. If such sections are not identified, this may lead to false assumptions and concluding remarks on signal quality and may also be a reason for signal anomalies and faulty measurements.