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The low-frequency oscillation (LFO) of electrified railway is largely related to the grid-side converters of the train. The mixed operation of different types of trains with different control methods and parameters of the grid-side converters brings new challenges to system stability. In fact, Dq-domain control (DQDC) and phase-domain control (PDC) are frequently used in train grid-side converters. This paper proposes a dq impedance modeling method of PDC train, which is in the same domain as the model of DQDC train, so that a unified train impedance model (UTIM) for the trains with these two control strategies can be established in dq-domain. Then, the generalized Nyquist stability criterion (GNSC) is used to study the LFO. The influence of the parameter on the system stability in the case of two types of trains connected is revealed, and will be different from the system with only one type of trains connected. Finally, time-domain simulations verify the correctness of the theoretical analyzing conclusions.