# Optical Fibres for Condition Monitoring of Railway Infrastructure--Encouraging Data Source or
## 4 Measurements and Results  
### Signal Pattern and Properties of Randomly Closen Channels (Location)  
In the context of railways, the altered backscatter of optical fibres deployed alongside railway tracks rises primarily due to train movements, third party intrusions and natural events or disasters. The question arises whether the altered backscatter shows the same signal pattern along the optical fibre or rather changes due to local conditions/boundaries and/or train movements. Naturally, minor changes in the signal pattern along the fibre are to be expected due to the properties of the fibre, but the extent to which this takes place and the reason for it need to be analysed. Figure 4 illustrates 60 s of the raw signal of five channels recorded with IU1. These five channels were randomly selected from a segment of 100 consecutive channels representing a 1 km long track section. The \(x\)-axis represents the time in seconds, whereas the \(y\)-axis represents the intensity of the backscattered signal in arbitrary units (a.u.). The train depicted moves from the right to the left and first crossed Channel 300 (grey) and Channel 399 (turquoise) at the end. This means that the train movement is measured opposite to the train direction. As illustrated, the signal pattern of each channel changed immediately as the train approached the specific point and altered the backscatter significantly. The dot-dashed lines represent the mean value for each specific channel. Although a certain jagged shape appears in each channel, caused by both the travelling train and the local conditions, the peaks, as well as the mean values, differ from each other enormously. While Channel 375 shows, together with Channel 399, the most distinct signal pattern and the highest peak dispersion, it also has the second-lowest mean value (dot-dashed line). The mean value depends on both the level of backscattered intensity before and after the train is visible in the channel and the scattering (jagged shape) taking place in the fibre  
Figure 3: **(a)** Cable duct parallel to track 1 guiding and protecting the optical fibre. There is usually only one cable duct, located on the left-hand side of track 1, even though it is mostly a double-tracked line; **(b)** in the case of multitracked sections, one cable duct is provided whereby the fibre layout may change significantly, especially in the vicinity of bridges and turnouts, resulting in considerable fibre length excess.  
during the train ride. The lower the backscattered intensity after and before a certain event occurs, the lower the mean value. This applies also for the signal pattern--the higher the peak dispersion, the lower the mean value, as the data sample consists of lower values.  
In contrast, Channel 300 shows not only the highest mean value, but also a significant scattering pattern. In addition, the levels of the unaffected signals, i.e., before and after the train approaches a certain point, differed significantly for each channel. This also resulted in notably varying means. The respective correlation between all channels is unambiguous, even though differing signal patterns were observed. The statistical values of the mentioned channels are listed in Table 2.