To improve the riding comfort of railway vehicles equipped with the air suspension system now in widespread use, we tested a semi-active air suspension control system with a variable orifice. The system is installed between the air spring and the auxiliary air chamber, and is adjusted using a controller with a design based on the H _∞ control algorithm. We carried out numerical simulations using a half railway vehicle model with a 4-element air suspension model, as well as performing excitation tests using a half carbody at a rolling stock test plant. The results show that the proposed system effectively reduces the power spectral density (PSD) of acceleration of the carbody floor. Additionally, little difference was observed between the vibration mitigation effectiveness of a reduced-order controller and that of the original one.