When a high-speed train (HST) enters a tunnel at speeds exceeding 400 km/h, the rapid increase in aerodynamic drag and the amplification of the micro-pressure wave (MPW) at the tunnel exit pose significant safety challenges. This study establishes a full-scale transient numerical model of a 3-car train and a double-track tunnel with a 100 m² cross-section. The effects of jet position (head, middle, or tail car) and dimensionless jet velocity ( \(\:{u}_{\text{j}\text{e}\text{t}}\) ), ranging from 0.1 to 0.7 of the train speed (U), on aerodynamic characteristics are investigated, revealing the control mechanisms and velocity effects of the airflow jet and suction method (AJSM). The results demonstrate a clear trade-off between drag reduction and MPW suppression. At \(\:{u}_{\text{j}\text{e}\text{t}}\) = 0.1 U, the middle car jet reduces the total drag by 5.6%, whereas the head car jet reduces the MPW by 37.7%. The head car jet configuration offers the best overall performance, lowering tunnel wall pressure by 22.0%, the average lateral force by 37.1%, and the peak wind velocity by 13.8%, thereby enhancing safety and comfort. As the U increases, the required \(\:{u}_{\text{j}\text{e}\text{t}}\) for effective MPW suppression follows a power-law growth, but the control effect saturates at 600 km/h, indicating a saturation of the control effect for the AJSM. The concept of a critical jet-speed ratio ( \(\:{R}_{\text{c}\text{j}}\) ) is introduced, providing practical guidelines for jet parameter selection. This research delineates the aerodynamic limits of the AJSM from a synergy-trade-off perspective and establishes design boundaries for active flow control in train-tunnel systems operating at 400–600 km/h.
