<p>Mud pumping under rail tracks was considered as a liquefaction or fluidisation process caused by the accumulation of excess pore pressure in subgrade soils. However, recent experimental evidences disapprove such an interpretation and support that it is a more general process of particle and pore fluid migration in soils subjected to dynamic loads. A layered gravel–sandy silt structure is used to investigate the suspension and deposition behaviour of particles under alternating dynamic and static loads. The results show that the migration behaviour of particles leads to the turbidity of slurry at different gravel depths fluctuates with time. The hydrodynamic response caused by slurry sloshing in the layered gravel–sandy silt structure is crucial in driving particle–fluid migration. The oscillating excess pore pressure creates a hydraulic gradient in the sandy silt layer, increasing the likelihood of fine particles detaching from the soil skeleton. A suspension criterion is established to quantitatively compare the suspension potential of particles of different sizes. The dynamic load intensity significantly influences the amount of particle migration and results in more particles achieving long-distance migration. These findings have direct and practical implications for the design and construction of rail embankments, particularly those built on or near ballast–subgrade interfaces.</p>