Ballastless track (i.e. slab track) systems are used extensively in passenger rail applications for improved track stability, alignment control, vibration, and life cycle cost (LCC) benefits. These systems regularly rely on Direct Fixation (DF) fasteners to connect the rail to the structure. Field performance observations have indicated that even under similar track geometry and train operating conditions, the DF fasteners useful life varies widely. Meanwhile, a review of literature reveals that there is limited prior research to guide optimization of DF fastener designs for heavy rail transit. Therefore, researchers at the University of Illinois at Urbana-Champaign (UIUC) conducted a field investigation at three sites on a United States legacy heavy rail transit system to quantify wheel-rail interface loading demands and DF fastener response. Track response variance across similar track geometry was found. Wheel loads ranged between 2.7 to 18.2 kip (12.0 to 81.0 kN) and 0.9 to 12.4 kip (4.0 to 55.2 kN) for vertical and lateral loads, respectively. Lateral rail head displacements ranged between −0.05 to 0.16 inches (−1.27 to 4.06 mm) while dynamic lateral stiffness ranged from 42 to 62 kip/in. (7.3 to 10.8 kN/mm), indicating a low stiffness ratio for the DF fastener studied. Differences in behavior are attributed to dynamic vehicle-track interaction, the relationship between balanced and operating speeds, and differences in track gauge between sites. A comparison of vertical loading results with two additional heavy rail transit agencies shows Burr distributions that accurately represent the loading demands. Results from this study provide quantitative information that can be leveraged to improve heavy rail transit DF fastening system design and development of representative design validation testing protocols.