According to the innovative technique features of high-speed bogies for European vehicles, an optimized design methodology for the rubber suspensions of under-floor masses was proposed based on rigid-flex coupling simulation techniques. Lateral vibration coupling mechanism is meant the coupling mechanism of lateral high-frequency vibrations caused by the lateral secondary suspensions in the interface of coach to running gear, and the three important particularities are as follows: anti-hunting high-frequency impedance, higher damping of coach yaw and the aluminum-alloy coach without longerons and skeletons. The modal vibrations of 1st order lateral bending mode in coach bottom will have therefore the very negative impacts on the technical specifications of 30-year life-time coaches. For the rubber suspensions of under-floor masses, the proportional damping is one of active factors in suppressing the lateral vibrations of under-floor masses. But there is optimal value for lateral acceleration of middle floor, i.e. the minimum value of (RMS)3 in full frequency-band can be achieved when the proportional damping is 0.5. Meanwhile the limits of the above damping technique shall be paid much attention, i.e. it is possible that the lateral coupling vibrations of under-floor masses will be produced under the exceptional conditions of running gear.