In order to improve vertical ride comfort aboard railway vehicles, it has recently become essential to suppress vertical bending vibration of carbody. In many cases, the resonant frequency of the system which consists of bogie frame and axle springs is close to the one of the first mode bending vibration of carbody. Therefore, suppressing the vibration of bogie frames near their resonant frequency effectively reduces the vibration of carbody. In this paper, we propose a method to suppress this vibration by controlling damping force of axle dampers, which are installed between bogie frames and wheel sets. The semi-active controller that is applied to determine the optimal damping force is designed based on the sky-hook control theory. Both numerical simulations using a vehicle model with 16 degrees of freedom and excitation tests using a carbody,with variable coefficient axle dampers on rolling stock testing plant are carried out. In results, we show that the control method effectively reduces the power spectral density (PSD) of acceleration on the floor and the ride comfort level (L_T) can be improved by about 3dB.