With the accelerated construction of electrified railways and the massive operation of new energy equipment, the broadband oscillations arising from the interactions between the power electronic converter of electric multiple unit (EMU) and the traction network threaten the safe and stable operation of the traction power supply system. The paper aims to evaluate small-signal stability and to identify the dominant parameters that lead to oscillations in the vehicle-grid coupling system. Firstly, a small-signal model is constructed, which includes grid-forming/following converters and d-q decoupling control (DQDC)-modeled trains. Furthermore, a state-space model of the coupling system is constructed, and eigenvalue analysis is employed to identify critical oscillatory modes. The modal participation factor is used to examine the dynamic interactions between EMU and renewable energy converters and to evaluate the influence of their control parameters on system stability. Finally, the accuracy of the proposed model and the validity of the theoretical analysis are verified through MATLAB/Simulink simulations, providing theoretical support for stability analysis in traction systems with high renewable penetration.