This article investigates the effect of vertical track stiffness, and its variations along the track, on track performance with focus on dynamic responses of the track due to parametric excitations. Two approaches for calculating global track stiffness, a static one based on Zimmermann's theory and a dynamic one based on the track model used in the dynamic vehicle-track interaction program Dynamisk Interaktion mellan Fordon och Farbana (DIFF), are discussed. A rolling stiffness measurement vehicle has been developed at Banverket, and measurements have been carried out over hundreds of kilometres of track. This article presents a statistical analysis of the collected results and provides information of track stiffness and its variation on typical Swedish tracks.