The structural vibration and re-radiated noise of viaduct due to railway traffic can be reduced by isolating the vibration energy transmission from the track to the viaduct. A frequency-domain model for analyzing the vibration power flow of the coupled railway vehicle-track-viaduct system is developed. This model consists of an analytical model of vehicle-track subsystem and a finite element model of viaduct, and only the motion in vertical direction is considered. The transmitted vibration power from the track into the viaduct due to the wheel/rail roughness is calculated, and the effects of the stiffness, loss factor of rail pad and the vehicle speed on the transmitted power are investigated. Numerical simulations show that the rail pad can attenuate the vibration energy transmission in the frequency range above the wheel-track resonance frequency for the direct fastening track on viaduct, and its stiffness has notable effects on controlling the re-radiated noise of viaduct. The overall transmitted vibration power level decreases by about 10 dB for a ten-fold decrease in fastener stiffness, and decreases by about 1.3 dB for doubling the loss factor of rail pad, whereas increases by approximately 6.6 dB per doubling in vehicle speed.