Rail ballast is an unbounded granular material that spreads laterally when subjected to train loading. Railroads can be reinforced by geogrids to reduce lateral movement and to optimize track performance. This paper presents a study of the behaviour of geogrid-reinforced ballast subjected to monotonic and cyclic loading using a large-scale direct shear box and a novel Track Process Simulation Apparatus (TPSA). The shear stress-strain response of fresh and fouled ballast reinforced by geogrid was investigated using large-scale direct shear tests subjected to normal stresses from 15 kPa to 75 kPa, where the levels of fouling varied from 20% to 95% Void Contamination Index (VCI). Cyclic tests for fresh and fouled ballast were conducted using the TPSA to realistically simulate real track conditions. The experimental results showed that a geogrid provides extra internal confinement and interlocks the aggreagtes in its apertures, hence reduces ballast deformation. The discrete element method (DEM) was used to model geogrid-reinforced fresh and fouled ballast subjected to monotonic and cyclic loading. Irregularly-shaped particles and geogird were simulated by clumping spherical balls together, while the coal fines were simulated by adding 1.5mm diameter spheres into the pore spaces of ballast. The predicted stress-displacement responses obtained from the DEM were in good agreement with those measured in the laboratory, where the peak shear stress of fouled ballast decreased and the dilation of fouled ballast increased with an increasing level of fouling. The contact force distributions and the orientations of normal and shear force were analyzed to provide more insight into the behaviour of ballast subjected to shearing.