This study presents an investigation into the mechanical behavior of geogrid-reinforced ballast subjected to cyclic loading focusing on the macro- and micromechanical features of the geogrid-ballast interaction mechanism. Key areas of interest include the effects of geogrid placement depth, aperture size, and stiffness on the motion of ballast particles, formation of contact force chains, and energy dissipation. A three-dimensional discrete element model, calibrated with experimental data, simulates ballast box tests performed on 300-mm-thick ballast layers reinforced by geogrids placed at depths ranging from 50 to 250 mm below the tie. The findings reveal that geogrids located within the upper 150 mm of the ballast layer significantly reduce tie settlement by minimizing particle movement, creating well-connected soil structures, and decreasing energy dissipation. Upon identifying 150 mm as the optimal geogrid placement depth, a parametric study evaluates the impact of the geogrid aperture size (A) and stiffness on the behavior of geogrid-reinforced ballast. The geogrid aperture size (A) is varied to give aperture size to ballast diameter (D) ratios ranging from 1.09 to 2.91, while the geogrid's stiffness ranges from 9.54 to 18.00 kN/m. Results indicate that A/D ratios greater than or equal to 1.45 are required for geogrids to perform satisfactorily, while stiffness appears to wield a negligible influence on the response of geogrid-reinforced ballast.