Mud pumping is an undesirable subgrade distress in ballastless high-speed railway, significantly affecting the ride comfort and posing a threat to train operation safety. In this study, a full-scale physical model of the ballastless slab track was developed. A rainfall simulator was installed, and various testing sensors were embedded in the trackbed to investigate the phenomenon of mud pumping in ballastless tracks. The results revealed a three-stage process for the intruded rainwater, including the initial vertical infiltration, the following horizontal infiltration, and the eventual roadbed saturation. A significant excess pore water pressure gradient (PWP) was created vertically in the roadbed due to the moving train loads. Similarly, a small longitudinal gradient was also observed. Both PWP gradients indicated the spatial migration of fine particles within the roadbed. The contact pressure distribution under the concrete base varied notably under different roadbed conditions. In the saturated state, the maximum dynamic soil stress, initially located at side of the concrete base transitioned toward the track center. The findings contribute to a deeper understanding of mud pumping mechanism in ballastless tracks.