In this paper, a modelling method of a 25 kV-50 Hz railway line, in a frequency range from 0 to 5 kHz, is presented. A model is proposed to quantify current and voltage harmonics generated by traction converters in different points of the network. An equivalent circuit, taking into account the skin effect for time-domain simulations, is also proposed. A new model, based on state space representation and transfer functions is developed to simplify the study of the interactions between several trains circulating on a line sector. As an example, the amplitudes of the harmonics generated by on-board active rectifiers are computed at several points of the network for a given topology of the overhead line and several positions of two trains on a sector with a junction.