Railway industries are placing greater emphasis on implementing fast and heavy haul corridors for bulk freight and commuter transport in order to deliver more efficient and cost-effective services. However, increasing dynamic stresses from the passage of trains progressively degrades and fouls the primary load-bearing ballast layer, which inevitably leads to excessive settlement and instability, damage to track elements, and more frequent maintenance. Ballasted tracks are subjected to even greater stresses and faster deterioration in sections where a reduced ballast thickness is used (e.g., bridge decks) or at locations where heavier concrete sleepers are used instead of lightweight timber sleepers. The inclusion of under sleeper pads (USPs) at the base of a concrete sleeper is one measure used to minimize dynamic stresses and subsequent track deterioration. In this study, cyclic loads from fast and heavy haul trains were simulated using a large-scale process simulation prismoidal triaxial apparatus (PSPTA) to investigate the performance of ballast improved by USPs. The laboratory results indicate that the inclusion of an elastic element at the harder interface of the concrete sleeper–ballast reduces the stresses transmitted to the ballast and the underlying layers and minimizes the amount of deformation and degradation of the ballast. A three-dimensional finite-element model was used to predict the behavior of ballast, and the influence of USPs on the stress–strain responses of ballast generally agree with the experimental findings.