## 4 Discussion  
In an undisturbed measurement, the single sensor achieves the smallest deviation from the METAS reference. In comparison to the single sensor, the accuracy of the chord method slightly decreases for an undisturbed measurement, which can only be achieved under laboratory conditions. This can be attributed to the transfer function of the chord method, and to mounting inaccuracies between the individual sensors, which in turn could lead to deviations in the measurement result. It is therefore important to ensure that the sensor plate in the measurement setup has close tolerances and a low thermal expansion over its length. It is noted that for selected one-third octave bands in the low wavelength range, a larger deviation from the reference is observed. This is also the case without interference at 10 mm, 20 mm and 25 mm. It should be investigated whether this is also the case on a smooth rail in real operation by measuring the running surface. The reference rail used has a ground and therefore rough surface. Stop points in the movement direction along the rail and subsequent acceleration have no significant effect on the measurement result, confirming the results of Grassie [21]. Heat application directed to the region between sensors and rail surface does not show any deteriorating influence on the measurement result. The changed optical conditions on the rail surface caused by the water worsen the measurement result. In particular for small wavelengths below 100 mm, an increased acoustic roughness level value is measured. For longer wavelengths, only a slight change can be detected compared to an undisturbed measurement. It is therefore recommended that measurements are carried out with dry rail conditions. This must be ensured either operationally by selecting the time and date of a measurement, or technically by removing the water. Similar as for the wet rail, levels at smaller wavelengths below 100 mm are influenced and increased by dust. Unlike water, the dust, solid particles or debris are not necessarily removed from the rail as the train passes over it and contribute to the acoustic emission from the wheel-rail interaction. It can be assumed that dust particles are usually displaced after they have been passed. Nevertheless, when operating on a train, it should be investigated to what extent dust particles pose a problem and whether further additional measures are necessary to remove them from the rail (e.g. by means of brushes). With a suitable mounting position on the train, the negative influence on the optical measurement concept could be reduced to a minimum. Depending on the type, size and amount of dust, the influence on the measurement result could vary considerably. To what extent and how often dust is a relevant obstacle for measurements on a train still needs to be investigated. Under the influence of a vertical displacement, the signal of the single sensor is not suitable for acoustic roughness measurements. Applying a filter is insufficient, since the frequency of the deflection can vary. The chord method in contrary can reproduce the profile correctly. The deviation from the METAS reference increases with increasing amplitude, and already deviates significantly at an amplitude of 2 mm in the wavelength range between 8 mm and 50 mm. The measurement result becomes unsuitable for the entire wavelength range with a further increase in amplitude. In the case of tilting, the signal of the single sensor also shows large deviations compared to the METAS reference. For the identical set amplitudes (as for displacement test scenario) of the pneumatic actuators, the mean deviations due to tilting are bigger than the ones caused by displacement of the sensor plate. The pneumatic actuators were not connected with the right or left end of the sensor plate, as shown in Figure 1. Therefore, with the same set amplitude, sensor 1 and sensor 4 are deflected more than in the case of uniform displacement of the entire sensor plate.