Noise emission is a major concern for residents living along railway lines. For a large range of conventional speeds, railway noise is dominated by rolling noise. The contribution of the track to rolling noise is mainly driven by the level of rail roughness and the Track Decay Rate (TDR). The MEEQUAI project (on-board measurement of the acoustic quality of the infrastructure) aims to develop a system capable of measuring these characteristics using on-board instrumentation. A state-of-the-art on the subject shows that indirect measurement from vibro-acoustic sensors like axle-box accelerometers or microphones in the bogie area seems possible, but that a number of improvements could be made to the existing methods, especially in estimating the transfer functions between the e_ective roughness and the signals provided by the sensors. The idea is to combine modelling and measurements to optimize the estimation of the transfer functions and the location of sensors while taking into account the variability of circulated tracks. Numerical analyses of transfer functions based on axle/track vibro-acoustic models are performed. Based on the simulations results, several improvements are proposed, with a view to future tests in rolling conditions.