## 5 Conclusions and Outlook  
An optical measurement system for measuring the acoustic roughness of rails was installed on a train. Measurements were performed at velocities up to 80 km h\({}^{-1}\). Promising results were shown, especially for wavelengths above 16 mm at a speed of 50 km h\({}^{-1}\). The average deviation from the reference was 1.48 dB for the entire wavelength range investigated at a speed of 50 km h\({}^{-1}\). The velocity of the train could be reliably calculated based on the measured sensor data. The train speed calculated from the laser sensors deviated by only 0.06 km h\({}^{-1}\) and 0.05 km h\({}^{-1}\) from the average speed measured on the train. Even in difficult environmental conditions with snow and moisture on the rail, it was possible to perform exploitable measurements. However, it was shown that the quality of the results for one-third octave bands is not adequate at smaller wavelengths. The influence of wetness on the quality of the results in the wavelength range between 3 mm and 16 mm significantly increased the mean deviation from 3.45 dB (wavelength range from 16 mm to 63 mm) to 6.44 dB (entire wavelength range) at a speed of 80 km h\({}^{-1}\).  
Further measurements should be carried out with a dry rail surface and at different speeds to determine the relevant influences for deviations in the spectrum. To improve performance at wavelengths below 16 mm, it is possible to reduce sensor distances by installing the sensors transversely to the direction of travel. This was not applied for these measurements due to the oval measuring point of the sensors. To correct dynamic influences, it is possible to install additional acceleration sensors which can remove vehicle dynamics from the measurement data. In addition, reproducibility should be investigated by measuring the same track section several times without extended intervals between the measurements. For this purpose, it is useful to have defined trigger marks to identify the measured segment more precisely. One possible approach could be to measure in parallel with a camera recording of the running surface and to flag the trigger marks optically. The measurement setup should therefore be supplemented by a camera. A camera can also be used to detect the lateral measurement position within the running surface. The influence of the lateral dynamics must be quantified and taken into account in the development of a positioning device. [PERSON] et al. (2018) developed a hunting motion compensation system that can operate up to a speed of 6 km h\({}^{-1}\). For the investigated and larger measuring speeds, the requirements are consequently more demanding, and a new solution may have to be found.  
Conceptualization, F.M.; Data curation, F.M. and R.W.; Formal analysis, F.M. and R.W.; Funding acquisition, M.K.; Investigation, F.M. and R.W.; Methodology, F.M.; Project administration, F.M. and M.K.; Resources, K.W.; Software, F.M. and R.W.; Supervision, M.K. and K.W.; Validation, F.M. and R.W.; Visualization, F.M.; Writing--original draft preparation, F.M.; Writing--review and editing, F.M., R.W., T.W., M.K. and K.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded (Grant number 1337000430) by the Federal Office for the Environment (FOEN) and the Federal Office of Transport (FOT). The APC was funded by ETH Zurich.  
Not applicable.  
The data used in this study are available from the corresponding author upon request.  
The authors would like to thank zb Zentralbahn AG for the realization of the measurement drive. The authors would also like to thank SBB, who carried out the reference measurements and provided the raw data.  
The authors declare no conflict of interest.