The paper describes the design and testing of an overvoltage damping filter supplied to the National Railways of Zimbabwe. The mechanism of overvoltage generation is explained, along with the need to limit overvoltage amplitude. An explanation of the choice of filter to reduce overvoltage is given, and the techniques used to derive the filter-component values and rating are described. A frequency-domain technique was derived to rate the filter components and this is different in some aspects from conventional filter-rating calculation methods. A Section covering protection and operating aspects is included, since many of these factors are unique to railway feeders. The paper terminates with a comparison between calculated rated values and measured filter currents and concludes that the methods chosen for the design are accurate.