This paper presents a methodology for the modelling of a 25 kV-50 Hz railway infrastructure, for frequencies from 0 to 5 kHz. It aims to quantify the amplifications of current and voltage harmonics generated by on-board converters into the infrastructure. A circuit is developed to model the skin effect in the overhead line for time-domain simulations. A new approach, based on state space representation and transfer functions, is also proposed to analyze the interactions between trains. The methodology is then applied in Matlab-Simulink and validated with EMTP-RV. Harmonic interactions between the infrastructure and on-board converters are analyzed. Then, an extension of the model is proposed for higher frequencies in order to study harmonic interactions between new on-board converters and the infrastructure.