In AC rail networks, the new locomotives using four-quadrant rectifiers are responsible for low frequency disturbances on overhead line voltage. In recent years, around the world, interactions between locomotives and power supply have been observed. In some sectors of the 25kV-50Hz French rail network, serious disturbances have affected the rail system operation while flicker was observed on the upstream electrical network. To prevent such dysfunctions, an analysis method is proposed. Any locomotive or multiple-unit train is characterized as an admittance matrix by using a Voltage Source Converter. After that, a stability study can be performed on each critical area with a view to determine the acceptable number of locomotives equipped with active front-end rectifiers.