Measurements of harmonics and waveforms were made in an electric system that feeds a railway network. The analyzed network is composed by cables in 20 kV and 6-pulse rectifiers, which are connected to a system in 132kV. The results of these measurements were used to adjust a digital model of this network. The purpose of this model, built with the ATP, was to determine the transient behavior that occurs during the harmonics filter operation. This filter was designed taking into account the harmonics found in the railway network.Instituto de Investigaciones Tecnológicas para Redes y Equipos Eléctricos (IITREE)