It is well established that rolling noise originates in the combined ‘roughnesses’ of the wheel and rail running surfaces. Through the rolling interaction of the wheel and rail this roughness imposes a time history of relative displacement across the wheel-rail contact that leads to vibration of the wheel and of the track. This vibration, in turn, gives rise to the noise components radiated by the wheel, the rail and the sleeper. The fact that at low (‘normal’) levels, the roughness gives rise to noise radiation linearly and accounts for the noise fully, has been shown by the comparison of theoretical models and carefully controlled measurements [1]. It has furthermore entered the practice of a number of railways to control the roughness, even of uncorrugated, track as a measure to reduce noise. In recent years, in line with the European Union’s strategy for harmonisation of internationally running train services in Europe, new Technical Specifications for Interoperability (TSI) have been written for the acceptance testing of new rolling stock. The acoustic TSI reflects the understanding of the noise generation mechanisms [2, 3]. In order to ensure that the acceptance test, that may be made at different locations on different rolling stock, is a fair test of the rolling stock and depends as little as possible on the local track design, the TSI specifies conditions for a ‘reference track’ on which pass-by noise measurements are to be made. The reference track is controlled in terms of the noise produced per unit level of combined roughness and the roughness of the rail head running surface. The first condition is characterised by a minimum decay rate spectrum that must be obtained on the reference track (for how this relates to the noise performance of the track see [4] and to [5] for the method of measurement). The second condition is a limit to the spectral level of rail roughness that may exist on the reference track [6]. To ensure comparable and...(...)...