This paper presents a study of the load-deformation behaviour of geocell-stabilised sub-ballast subjected to cyclic loading using a novel track process simulation apparatus. The tests were conducted at frequencies varying from 10-30 Hz. This frequency range is generally representative of Australian Standard Gauge trains operating up to 160 km/h. The discrete element method (DEM) was also used to model geocell-reinforced sub-ballast under plane strain conditions. The geocell was modelled by connecting a group of small circular balls together to form the desired geometry and aperture using contact and parallel bonds. Tensile and bending tests were carried out to calibrate the model parameters adopted for simulating geocell. To model irregularly-shaped particles of sub-ballast, clusters of bonded circular balls were used. The simulated load-deformation curves of the geocell-reinforced sub-ballast assembly at varying cyclic load cycles were in good agreement with the experimental observations. The results indicated that geocell decreased the vertical and lateral deformation of sub-ballast assemblies at any given frequency. Furthermore, the DEM can also provide an insight into the distribution of contact force chains, and average contact normal and shear force distributions, which cannot be determined experimentally.