Large-scale cubical triaxial tests were conducted to investigate the behavior of reinforced and unreinforced subballast under cyclic load. Granular material with an average particle size (D50) of 3.3 mm and geocell with a depth of 150 mm and nominal area of 46×103 mm2 were used in this study. The laboratory results proved that subballast stabilization was influenced by the number of cycles (N), the confining pressure (σ′3), and the frequency of train-caused vibration (f). The experimental results also confirmed that the geocells influenced the subballast behavior under cyclic loading, particularly at low confining pressure and high frequency. The additional confining pressure induced by the geocell reduced its vertical and volumetric strains. The optimum confining pressure required to reduce excessive volumetric dilation also was identified in this study. An empirical model using a mechanistic approach is proposed to determine the additional confinement induced by the geocells, as well as the practical implications of the experimental outcomes.