A straightforward and practical method of frequency-domain analysis is developed for coupled vehicle-track system. The influence of the track flexibility and spatial coherence of irregularities on frequency response of vehicle-track systems are systematically studied accounting for train velocity and irregularity wavelength. Calculations show that the track flexibility cannot be ignored to obtain an accurate response of wheels whereas the resonance frequencies of car body motions remain unchanged. The inclusion of track flexibility enables consideration of wave reflections in rail sections between different wheels. The excitations at different wheels due to irregularity have phase lags determined by the train velocity and distances between wheels. This spatial coherence is important to the system response. The influence of contact spring on the system frequency response is examined. It is found that the system response converges at a certain value of the contact stiffness and the track stiffness governs the wheel-rail interaction after then.@en