Rail pad is an important component in ballasted railway track. Its main function is to attenuate track loads, preventing underneath railway sleepers from excessive stress waves. Generally, the dynamic design of tracks relies on the available data that are mostly focused on the condition at a specific toe load. Recent findings show that track irregularities could significantly amplify the loads on railway tracks. This phenomenon gives rise to a concern that the rail pads may experience higher effective pre-loading than anticipated in the past. On this ground, this paper highlights the significance of accounting for effects of pre-loading on dynamic properties of polymeric rail pads. An innovative test rig for controlling preloads on rail pads has been developed. Based on single-degree-of-freedom impact-excitation responses, the rail pads have been tested to determine their modal parameters such as dynamic stiffness and damping constants in the laboratory. The influence of large preloads on dynamic properties of the rail pads has been identified. The design relationships have been proposed that account for the influence of the level of preload on the dynamic properties of the components of railway tracks.