# Optical Fibres for Condition Monitoring of Railway Infrastructure--Encouraging Data Source or
## 4 Measurements and Results
### Signal Pattern and Properties of Two Trains in the Same Composition at the Same Channels (Location)
Firstly, frequencies up to approximately 40 Hz are steadily present even without a train movement as is clearly visible and thus frequencies up to 0.04 kHz can be classified as noisy (continuously present over the entire time). When a train approaches and passes the specific channel between second seven and 14, as displayed in Figure 6a, higher frequencies were also present and emerged due to the train's sound and vibrations. A closer look at the frequency spectrum reveals the impact of at least each bogie emphasised by the \"yellowish\" peaks. The frequency spectrum of channel 300 for Train Journeys 1 and 2 is almost the same, meaning that the system measures the same regardless of the Train Journey. This is confirmed by the results of the other channels as a similar effect can be observed for the other channels too. Another very interesting result relates to the time-domain analyses of the backscattered intensity (see figures before) and the frequency content of the respective channels (see figures below). The signal would appear to be relatively more stable in the frequency domain than in the time domain, which is underpinned by the results illustrated in Figure 6. These only minor changes in the frequency properties, of course, can occur due to various reasons but, the signal seems to be more reliable in the frequency domain. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & \multicolumn{8}{c}{**Raw Signal**} \\ \cline{2-10}  & \multicolumn{4}{c}{**Train Ride 1**} & \multicolumn{4}{c}{**Train Ride 2**} \\ \cline{2-10}  & **300** & **325** & **350** & **375** & **399** & **300** & **325** & **350** & **375** & **399** \\ \hline Min (a.u.) & 8546 & 15,346 & 10,245 & 9708 & 5593 & 9521 & 18,949 & 11,869 & 13,022 & \(-4552\) \\ Max (a.u.) & 24,299 & 20,910 & 23,683 & 24,638 & 24,140 & 24,578 & 23,491 & 24,093 & 24,809 & 24,369 \\ Mean (a.u.) & 19,355 & 18,316 & 21,407 & 18,383 & 19,948 & 20,206 & 21,845 & 19,930 & 21,720 & 18,949 \\ SD (a.u.) & 1496 & 509 & 1392 & 2380 & 1898 & 1344 & 525 & 1128 & 1042 & 2835 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Statistical values of the measurement as two trains in the same composition pass through channels 300, 325, 350, 375 and 399 on the same day. Figure 6: _Cont._  
The train type, condition and speed undoubtedly affect the signal pattern, scattering behaviour and, consequently, the frequency content. Equally important, however, is the influence of the infrastructure and its condition. External perturbations and local conditions change the signal properties providing the user with essential information on boundary conditions. Otherwise, the sensing system could not be used for condition monitoring, intrusion monitoring and many other applications. Ensuring the pipeline integrity, the detection of immediate leakages and risk mitigation, for example, are very common applications for DAS, because pipelines are linear assets. The influence of the infrastructure on the signal properties can best be demonstrated by simply comparing the results depicted in the figures above (Figures 5 and 6). Each channel corresponds to a certain position of the monitored track. Channel 300, depicted in Figures 5(a) and 6(a),b, are linked to a straight section with concrete sleepers. The scattering is quite characteristic and shows a distinctive pattern. As mentioned previously, the train-related signal content and noise were visible up to frequencies of 300-400 Hz, but the impact of the vehicle and the response of the infrastructure are very clear. The locomotive is the heaviest vehicle in the train composition, but shorter bogie distances made it difficult to identify the impact of each axle. Nevertheless, at least two high values were visible. The bogie distance was greater for the coaches, and they are thus more easily recognized. When compared to the results above, the scattering  
Figure 6: Spectrograms of two train journeys in the same composition in 5 consecutive channels distributed equidistantly.