# 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)
(**a**) Channel 300, Train journey 1; (**b**) Channel 300, Train journey 2; (**c**) Channel 325, Train journey 1; (**d**) Channel 325, Train journey 2; (**e**) Channel 350, Train journey 1; (**f**) Channel 350, Train journey 2; (**g**) Channel 375, Train journey 1; (**h**) Channel 375, Train journey 2; (**i**) Channel 399, Train journey 1; (**j**) Channel 399, Train journey 2.  
in Channel 325 and 350 (Figure 6c-f) changes significantly. However, when only 250 and 500 m away from Channel 300, the two measurements show no correlation. A question then arises regarding the reasons for the drastically changed measurement results. This issue can be resolved by linking the asset data, both the fibre and track layout and channel position. Channel 325 lies within a station and Channel 350 is in the middle of a coil where both seem to have no sensing capabilities at all. The scattering occurring at a position only 250 m further away (again straight section and concrete sleepers) once more delivers a completely different signal pattern (Figure 6g,h). This channel showed not only a very prominent peak dispersion, but also seemed to be very noisy. The same applies for the scattering observed in Channel 399. In contrast to Channel 300, neither the number of vehicles nor the impact of single bogies or axles could be identified. In this case, the train ride is visible, and the signal was constantly present up to a frequency of \(\sim\)0.4 kHz. Higher frequencies were hardly present though. Signal Pattern and Properties When a Train Passes an Insulated Rail Joint in an Advanced State of Wear  
The last position analysed was linked to an insulated rail joint (IRS) showing an advanced state of wear. The measurements revealed a scattering completely in contrast (Figure 7) to those analysed previously. Firstly, no extensive peaks could be observed when analysing the backscattered signal. In fact, the scattering showed no characteristic properties and the signal seems to be weak. While high peak values could previously be identified, this was no longer the case at this position. A reason for this could be that it is almost the 20 th km of IU2. However, an already weakened signal after only 20 km does not appear to be a very reasonable assumption. On analysing the spectrogram of the channel, the picture completely changes. Before the train approached and passed the channel, frequencies up to approximately 0.04 kHz were steadily present containing no valuable information. This corresponds very well to the results obtained previously. At the moment the train entered the channel, the backscattered signal was altered, and the frequency content changed (Figure 7b). In this specific case, higher frequencies were observed as a result of the infrastructure response to the train passing the worn-out rail joint. The first yellowish peaks arose due to the impact of the locomotive's bogies and axles followed by the coaches it was pulling. Higher frequencies were no longer present as the train travelled further and no axle passes the specific point. Consequently, frequencies up to \(\sim\)0.3 kHz only were visible and can be considered as the train specific signature. As the next axle and bogie passed the rail joint, immediate responses to the impact and higher frequencies were present again. While the raw backscattered intensity has a remarkable behaviour, the filtered signal exposes a different picture. The use of a Butterworth high-pass filter (HPF) third order with a 400 Hz cut-off frequency removed all lower frequencies, allowing only frequencies higher than 400 Hz to pass. The impact of not only each bogie but rather of every single axle was unambiguously identifiable and the vehicles forming this train were countable, while it was also evident that all noise has been removed by applying an HPF (Figure 7c). Each time an axle or at least a bogie passed the rail joint, the infrastructure reacted to the impact of the load. The depicted train formation consisted of a locomotive and nine coaches and is, in fact, the reference train.