# Optical Fibres for Condition Monitoring of Railway Infrastructure--Encouraging Data Source or
## 5 Conclusions and Discussion  
Utilising and adapting existing infrastructure--in this specific case, the infrastructure of deployed optical fibres alongside railway tracks--for sensing and consequently monitoring the condition of railway infrastructure represents a promising data source, for infrastructure managers to increase the information volume and gain knowledge about their asset condition. The network of installed optical fibres covers wide parts of the infrastructure primarily used for telecommunication and signalling to date. Hence, connecting one dark fibre to an interrogator unit discloses all relevant events taking place on or in the proximity of linear infrastructures. This already constitutes a valuable information source for infrastructure managers since it enables them to monitor the infrastructure in question in real-time and can react immediately in case of disruptions. The sensing principle is a reaction to external perturbations resulting in an altered backscatter which makes it possible to conclude whether an unusual event has occurred at a specific position. The results outline the enormous potential of the sensing system but continued serious efforts must made in order to derive valid results. In general, the results presented confirm the system's capability to track the condition monitoring and identification of isolated track defects. The fibre is highly sensitive to external perturbations leading to significantly different scatterings taking place within the cable. This shows that it is of crucial importance to know the root cause of the changed scattering processes and where they occurred along the track. Therefore, the identification of the optical fibre cable layout and its link to the track is mandatory in any case regardless whether using already installed fibres or solely for railway infrastructure condition monitoring deployed ones. Nevertheless, optical fibres alongside railway tracks do not always run parallel to the track because the distance between the cable guiding  
Figure 7: Raw backscattered signal (**a**), spectrogram (**b**) and filtered signal (**c**) of a train passing a rail joint in an advanced state of wear. duct and the track varies. In addition, the fibre layout changes in complex areas such as stations or multitracked sections. The FOS signal is affected by several circumstances changing both the sensing properties of the system and the scattering. One reason can be found in the sensing set up, because a dark fibre of already installed optical cables is utilised. A modified and customised experimental arrangement may lead to less random signal patterns and thus to more precise measurements. Nevertheless, some hurdles can be overcome by developing a methodology which allows a like-with-like comparison of and finally drawing consistent conclusions on the system's capability for infrastructure condition monitoring. An example of a feasible approach to tackle this issue is presented here where the results presented so far proved that the measurement method can derive valuable information on areas of interest. As shown, each train type produces a different signal pattern leading to a specific signal characteristic. The FOS signal changes in a remarkable manner over a given time as well as within a day of two train journeys in the same train composition. This may happen due to several reasons/conditions and some of these can be overcome with a methodology that always enables a like-with-like comparison. An effort to classify these influences is made as follows:  
1. Infrastructure condition. 2. Location, position and fixation of optical fibre cable. 3. Properties of optical fibre cable. 4. Train condition/properties. 5. Random effects. 6. Sensing properties. The location, position and the type of fixation of the optical fibre certainly influence the sensing properties as well as the properties of the fibre. While these parameters are closely linked to the fibre itself, further parameters are assigned to both infrastructure and trains.