A method will be presented which allows a construction of an acoustic source model based on the analysis of microphone array measurements during a train pass-by. The conventional array beam-forming technique is used as a kind of pre-processing or "rst order analysis to calculate in a second step the true source strengths by a back-projection method. The "nal equation to be solved connects the source power distribution on the vehicle surface with the measured microphone array amplitudes via a Green's function transfer matrix. By these means the side-lobe e!ects of the array characteristics as well as the Doppler frequency shift and amplitude augmentation of the sources are removed. The model is updated for consecutive time slices during the pass-by so that it is possible to study directional and time-dependent e!ects on the source characteristics. The method has been applied to measurements on the ICE-V train where three known arti"cial noise sources (loudspeakers) had been mounted on the outside of the sidewall. The source power distribution function gives a much better source separation than the conventional beam-forming result. The calculated source strengths show a good agreement with the true values.