In North America, most rail corridors are constructed using ballasted track. Monitoring the ballast support condition and improving upon current sub-structure/ballast maintenance strategies will ensure safe railroad operations. However, it is inherently difficult to evaluate the pressure distribution at the sleeper-ballast interface. Researchers at University of Illinois at Urbana-Champaign (UIUC) have developed an instrumentation strategy and analysis tool, the support condition back-calculator, to quantify ballast pressure distributions under concrete sleepers without interrupting the traffic. This laboratory-validated non-intrusive method uses concrete sleepers’ bending moment profile and rail seat loads as inputs to back-calculate the reaction distribution through the use of an optimization algorithm. To better understand the in-service ballast support conditions, this technique was deployed in the fieldona Class I heavy haul freight railroad in the United States. Concrete surface strain gauges were installed on concrete sleepers to measure their in-field bending moments. Wheel Impact Load Detectors (WILD) were used to measure rail seat input loads. The focus of this paper is to quantify the ballast pressure distributions beneath concrete sleepers on the heavy-haul tangent track. An evaluation of ballast pressure distribution variations between adjacent sleepers and through tonnage accumulation are also included. The information presented in this paper demonstrates the potential of the back-calculator for monitoring the ballast condition and will assist the rail industry in optimizing tamping cycle, enhancing safety, and developing more representative sleeper flexural designs for North America railroad applications.