###### Abstract  
A large part of the noise emissions from rail traffic originates from rolling noise. This is significantly determined by the surface roughness of the wheel and the rail. To quantitatively assess the noise generation from the wheel-rail contact, it is necessary to measure the surface roughness of the rail network. Direct measurements via trolley devices are usually associated with the need for a free track and limitation in velocity. Indirect measurements of rail roughness, such as measuring achex-box accelerations, enable operation during regular passage but only estimate the acoustic roughness. In this study, the potential of an optical and consequently contact-free measurement method using laser triangulation sensors to measure rail roughness from the train is investigated. The approach can combine the advantage of operation during regular passage with the characteristics of a direct measurement, enabling large-scale monitoring of the rail network. A measurement run with a train was carried out on a meter-gauge track at speeds up to 80 km h\({}^{-1}\) The results of the optical measurement approach were compared with a tactile reference measurement on the track. The results show good agreement of the new measurement setup for dry rail surface conditions at 50 km h\({}^{-1}\), with a mean deviation of 1.48 dB.  
Keywords:railway rolling noise; rail profiles; acoustic roughness; condition monitoring; chord method +
Footnote †: journal:  
a speed of 0.5 m s\({}^{-1}\). Higher speeds can be achieved with on-board measurements using indirect measurement methods. [PERSON] [12] described the measurement of a train's axle box acceleration and determined the rail profile. [PERSON] et al. [13] stated that this approach depends on track construction and the speed of the train. [PERSON] et al. [14] also considered the influence of wheel roughness in the evaluation. Alternatively, to measuring axle-box accelerations, the noise level close to the rail can be measured. [PERSON] et al. [15] described a measuring wagon equipped with this setup. [PERSON] et al. [16] described the need for appropriate system calibration, which depends on track properties. According to [PERSON] et al. [17], a temperature change can already influence the rolling noise generated.  
By implementing an optical measurement with laser triangulation sensors, it is possible to measure rail roughness directly and without contact. This makes it possible to combine the advantages of direct measurement with those of indirect measurement. Measurement can be carried out without calibration measurements and at the line speed of a measuring vehicle. The functionality of the approach was demonstrated by [PERSON] et al. [18] in laboratory tests on a test bench based on the chord method described by [PERSON] [19]. In laboratory tests, only low train speeds and artificial external disturbances of the train were tested. In this study, the application of a chord method approach based on optical sensors is tested on a moving train (meter-gauge track) at a driving velocity of up to 80 km h\({}^{-1}\). In addition, it is shown that the velocity of the train can be determined from the sensor signals since multiple optical sensors measure the longitudinal profile on the same line. To classify the performance of the optical approach, a reference is taken with an RM 150 HR instrument from Vogel & Plotscher. The acoustic roughness is specified and compared in terms of the acoustic roughness, as described in EN 15610 [20]. Improvements with respect to the installation and operation of the system on a train are highlighted.