Geogrids have been successfully used in highway applications for stabilization and reinforcement purposes. Recent research efforts have also been dedicated to study geogrids in railway applications, especially for ballast reinforcement. Ballast, typically containing large-sized aggregate particles with uniform gradation, is an essential layer in railway substructure to facilitate load distribution and drainage. However, the reinforcement effect of geogrids in ballast has not yet been thoroughly investigated. Especially with the accumulation of tonnage, ballast will increasingly get fouled due to aggregate degradation or contamination by other materials, affecting aggregate-geogrid interlock mechanism at different levels of degradation. In this study, monotonic triaxial strength tests - performed on both clean and fouled ballast specimens - have been reinforced by geogrids and tested using a large-scale triaxial test device recently developed at the University of Illinois specifically for ballast size aggregate materials. Two different geogrids with square- and triangular-shaped apertures are used in comparison. To further investigate the geogrid reinforcement mechanisms, an imaging based discrete element modeling (DEM) approach was also adopted with the capability to create actual ballast aggregate particles as three-dimensional polyhedron blocks having the same particle size distributions and imaging quantified average shapes and angularities. By addressing adequately the particulate nature of different sized and shaped ballast aggregate particles and their interactions with each other at contact points, and through the innovative use of membrane elements surrounding the cylindrical ballast specimen, the ballast DEM model accurately captured the strength behavior of both clean and degraded ballast specimens reinforced by geogrids with different aperture shapes.