Suppression of the vertical bending vibration of carbodies has recently become essential in improving the riding comfort of railway vehicles. In many cases, the resonant frequency of the system (consisting of a bogie frame and axle springs) is close to that of the first mode bending vibration of the carbody, so suppressing the vibration of bogie frames near their resonant frequency effectively reduces carbody vibration. In this paper, we propose a method of suppressing such vibration by controlling the damping force of axle dampers installed between bogie frames and wheel sets. The design of the semi-active controller applied to determine the optimal damping force is based on the sky hook control theory. Numerical simulations using a vehicle model with 16 degrees of freedom as well as excitation tests using a carbody with variable axle dampers at a rolling stock test plant were carried out. The results show that this control method effectively reduces the power spectral density (PSD) of acceleration on the floor and that the riding comfort level ( L_T ) can be improved by about 3 dB.