The authors are developing a vibration evaluation system, which consists of field-portable exciters (actuators) and data processing software, to estimate vertical vibration and ride quality of railway vehicles. One of the main characteristics of the system is that the acceleration power spectral density (PSD) at arbitrary point on the carbody in running condition is synthesized numerically from measured data obtained when each axle box or each wheelset is excited individually using limited number of actuators. This paper focuses upon the data processing method for the vibration evaluation system. A series of excitation tests for a commuter vehicle is carried out to confirm the basic concept to estimate the PSD by using the rolling stock testing plant at Railway Technical Research Institute. Some excitation methodologies and data processing formulae are compared among each other. It is clearly shown that the estimated PSDs well agree with actual PSD. It is found out that the estimation gives high accurate results when only excited axle box acceleration are used in estimation analysis.