## 4 Discussion  
The optical measurement concept showed promising results, especially for dry/drier rail sections measured at a speed of 40 \(\mathrm{km\;h^{-1}}\) and 50 \(\mathrm{km\;h^{-1}}\), respectively. For the wavelength range below 20 mm, an increasing deviation from the reference can be observed. These deviations increased for the measurement at 80 \(\mathrm{km\;h^{-1}}\). The reasons for this could be the wet rail during the measurement and the influence of vehicle speed in the form of disturbances. Both influences could not be separated from each other at this point. To perform a comparison between optical measurement and a tactile reference in the wavelength range between 63 mm and 250 mm, a longer measurement length of the tactile reference measurement would be required. The optical device performed continuous measurements and was therefore not limited in measuring length. For validation, a trolley device would be suitable. The external disturbances of the train prevent the post-optimization of the measurement results because individual chords in the measurement setup are more disturbed than others. The lead sensor in the direction of travel failed before the tunnel section due to the wetness, which reduced the number of chords to one. The best results can be obtained in comparison with segment B of the reference section. Nevertheless, a lateral offset to stylus 2 as well as a longitudinal offset was expected, since no trigger marks were available. The spectra of stylus 1 and 3 represent the range of variation in the lateral direction. As the measurement on the reference section could only be carried out once, no information on the repeatability of the measurement system could be obtained. Snow accumulation and wetness caused an influence on the measurement results. Due to increasing accumulations, the signal of the first sensor in the direction of travel was unreliable at the beginning of the tunnel section. Figure 9 shows the accumulations and the ambient conditions on the day of the measurement run. Measurements with this optical measurement set-up should not take place under the described weather conditions. To keep the device operational between measurements, measures, as described by [PERSON] et al. [24], may be necessary. In addition, wetness created by the wheels of the train throwing snow and water caused further disturbances in the measuring system. This applies in particular to the first sensor in the direction of travel. Deviations in the measurements from the reference increased greatly in the wavelength range below 16 mm under the influence of wetness. In addition, there were clusters of  
Figure 9: Snow accumulation at the measuring box and driving in snow (snow cover thickness approx. 50 cm). outliers in the measured data set, which were not detected by the outlier removal and which distorted the spectrum of the acoustic roughness or shifted it to higher levels. The measuring distance of 35 mm between the sensor lenses and the top of the rail may be too small for regular use with other vehicles. Depending on the installed railway infrastructure, collisions can occur at this height. An example of this could be switches on a track with a toothed rack. In addition, the spring deflection of the primary suspension stage is often not known in detail and depends on the state of the loading of the vehicle. Based on the investigations of [PERSON] et al. [25], a deflection in the range of 60 mm to 70 mm is to be expected. The sensor model used in this study had a measuring range of 10 mm. The limit of the measuring range is reached at points of interference that cause greater deflection, which can be obtained from the raw data. It is therefore recommended to use sensors with a higher measuring distance and a larger measuring range. In the selected measurement setup, it was not possible to subsequently adjust the lateral position of the system during the current measuring ride. The hunting motion of the train on the track must be considered as a source of interference for the measurement.