Abstract: The frequency domain impedance/admittance model plays an important role in analyzing the oscillation of electrified railways. However, the traditional impedance model usually ignores the line impedance between locomotives when analyzing multiple locomotives, and the simple modelling of grid side impedance leads to a large error in stability analysis. In addition, when analyzing the through power supply mode, the error is more obvious because the source/load impedance relationship is simply calculated by parallel analysis. The traditional impedance modelling method ignoring line impedance cannot deal with the stability analysis of multi-nodes and multi-locomotives connected to the grid under through power supply mode. Therefore, a small signal multiport equivalent modelling method is proposed in this paper. First, according to the small signal multiport equivalent modelling theory, the impedance in the system is divided into active locomotive impedance and passive line impedance. The mathematical model of the open-loop gain matrix is deduced, and the stability is determined by the generalized Nyquist stability criterion. Then, the stability and sensitivity of different power supply modes under typical working conditions are compared and analyzed, and the effects of the power supply system and train system on train-network stability are discussed. Finally, the theoretical analysis is verified by simulation. The results show that the multiport equivalent modelling method has better generality and accuracy than the traditional impedance modelling method.