<p>In recent years, train-tail swaying of 160 km/h electric multiple units (EMUs) inside single-line tunnels has been heavily researched, because the issue needs to be solved urgently. In this paper, a co-simulation model of vortex-induced vibration (VIV) of the tail car body is established, and the aerodynamics of train-tail swaying is studied. The simulation results were confirmed through a field test of operating EMUs. Furthermore, the influence mechanism of train-tail swaying on the wake flow field is studied in detail through a wind-tunnel experiment and a simulation of a reduced-scaled train model. The results demonstrate that the aerodynamic force frequency (i.e., vortex-induced frequency) of the train tail increases linearly with train speed. When the train runs at 130 km/h, with a small amplitude of train-tail swaying (within 10 mm), the vortex-induced frequency is 1.7 Hz, which primarily depends on the nose shape of the train tail. After the tail car body nose is extended, the vortex-induced frequency is decreased. As the swaying amplitude of the train tail increases (exceeding 25 mm), the separation point of the high-intensity vortex in the train wake shifts downstream to the nose tip, and the vortex-induced frequency shifts from 1.7 Hz to the nearby car body hunting (i.e., the primary hunting) frequency of 1.3 Hz, which leads to the frequency-locking phenomenon of VIV, and the resonance intensifies train-tail swaying. For the motor vehicle of the train tail, optimization of the yaw damper to improve its primary hunting stability can effectively alleviate train-tail swaying inside single-line tunnels. Optimization of the tail car body nose shape reduces the amplitude of the vortex-induced force, thereby weakening the aerodynamic effect and solving the problem of train-tail swaying inside the single-line tunnels.</p>