A solution procedure is presented to compute free field vibrations induced by train or road traffic, the excitation being either deterministic or stochastic. The full interaction between the vehicle, the track or road and the soil is accounted for, using a substructure approach that takes advantage of the fact that the properties of the track or road and the soil do not change along the longitudinal direction. A time-frequency approach is proposed to characterize the free field radiated in the soil. The examples show the importance of guided waves along the track for understanding the dynamic behavior of the track or the road.