Following an extended period of service or exposure to geohazards, the ballast beneath certain sleepers undergoes deformation, leading to complete separation from the bottom of the sleeper, referred to as an unsupported sleeper. This study delves into the dynamic response characteristics of both supported and unsupported sleepers in ballasted tracks, employing a moving vibrator. Specifically, a 1/9-size track model was meticulously crafted in the laboratory, encompassing 21 sleepers, 2 rails, and a ballast bed with a density of 1.6 g/cm3, with the aim of emulating Japanese high-speed railways. The investigation focused exclusively on scenarios where the central sleeper existed in an unsupported state. The mobile vibrator, outfitted with a pair of unbalanced 10 g weights rotating in opposite directions, applied cyclic loading solely to rails in the lateral direction. Accelerometers affixed to both the sleepers and vibrator meticulously monitored dynamic acceleration and displacement in the lateral direction. The lateral displacements of the vibrator and both supported and unsupported sleepers were meticulously averaged within a peak interval time zone of 0.1 s as the vibrator traversed them. Furthermore, the study identified displacement relations between sleepers and the vibrator by scrutinizing the ratio of lateral displacements of the sleeper to those of the vibrator. The outcomes elucidated distinctions in lateral displacement relations between the unsupported and supported sleepers. Moreover, these relations exhibited variability contingent on the depth of the gap for the unsupported sleeper. These findings posit the feasibility of promptly discerning sleeper-supported conditions through an in-depth analysis of the dynamic response exhibited by the vibrator and sleeper.