Internal instability occurs when the finer fraction of a filter escapes with the infiltrates, causing permanent changes to its particle size distribution and rendering it ineffective. Thus far, numerous criteria based on particle and constriction size distributions have been proposed to assess the potential of internal instability. This paper reports the results of pressure gradient controlled hydraulic tests performed on granular soils including silt-sand-gravel, sand, and sand-gravel mixtures with their uniformity coefficients ranging from 1 to 304 and compacted at varying relative densities between 0 and 100 %. Select dense samples were then subjected to a series of hydraulic tests under static loading up to 100 kPa and cyclic loading up to 30 Hz, simulating subballast filtration under heavy-haul freight trains with speeds as high as 210 km/h. The analysis revealed that the agitation and pore pressure development under cyclic loading promotes premature suffusion, thereby making it the worst case of filtration. An objective evaluation of some of the existing criteria led to a more realistic interpretation of experimental results based on a modified technique that accurately assessed the internal stability of soils under cyclic loading. Results of internal stability assessments from existing geometrical criteria were then compared with those from the proposed technique, which showed enhanced success, thereby contributing to increased confidence in the practical design of granular filters under cyclic conditions.