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The AC 25 kV electric railways with DC drive locomotives are one of the main harmonic pollution sources of electric power grids in China. To assess the power quality of the railway supply networks needs to model not only the fundamental currents of the trains but also their harmonic contents. It is difficult to simulate accurately the dynamic current waveforms of an electric locomotive running on real lines because there are too many random or uncontrolled factors that can intervene the working state of the main power circuit of the locomotive. This paper presents a modelling method which can be used to imitate the probabilistic property of the current waveform and can be embedded in the train simulator. This method is based on statistical analysis of a large amount of measured data of locomotive currents that were acquired on actual railway line. A computer program has been developed for the 8K locomotive, a kind of widely used locomotives for freight services in China. The program has been applied to assess the power quality of railway power supply utilities, which shows that this modelling method can give a very realistic result from the statistics point of view and has a fast speed in the simulation.