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AC railway traction systems are particularly susceptible to harmonic distortion. They are weak single-phase networks often loaded with severely distorting SCR-based locomotive drives which continually change their point of physical connection to the network. For a public distribution system the total harmonic distortion is a clear measure of supply quality. However, for a traction system the more important parameter is the voltage form factor, which is directly related to the maximum power a locomotive can deliver. To minimize this form factor, this paper proposes the use of a shunt active filter that minimizes the third, fifth, and seventh harmonic voltage components at the point of filter coupling. It is shown that the filter is effective in both compensating the pantograph voltage form factor, and providing reactive power to maintain the feeder voltage. The results are confirmed by detailed simulation using the EMTDC package.