Primarily being employed in ballasted bridge decks and tunnels, under-ballast mat applications have seen growth in the North American market as a solution to reduce the stress state of the ballast by lowering the track stiffness and reducing ground-borne vibrations. However, the German DIN 45673 standard is currently the sole reference procedure for quantifying under-ballast mat’s mechanical fatigue performance by using real ballast particles as a loading contact interface and is tailored to European loading characteristics. This presents challenges for testing materials intended for North American heavy haul freight lines implementing such procedures. Moreover, large intrinsic variability is present when employing the real ballast material as a loading contact interface. This work presents findings from laboratory mechanical fatigue experiments conducted on under-ballast mat samples based on DIN-recommended procedures. Two loading contact interfaces—ballast box and geometric ballast plate—and two load magnitudes—representing European mainline and North American heavy haul freight loading environments—were applied to the samples in this study. For all tests, no significant physical sample damage was observed after testing. Further, although significant immediate change in bedding modulus was observed for the North American-loaded samples, modulus changes in both test cases were similar when evaluated one week after the completion of the tests. In addition, gradation analysis of the ballast material revealed little impact other than particle surface wear. Finally, results from this work demonstrate that geometric ballast plate is an effective alternative for investigating under-ballast mat’s performance and should be useful for the future development of North American-recommended practices for testing under-ballast mats subjected to heavy haul loads.