Under-ballast mats (UBMs) have become more popular recently in railroad track engineering. Benefits of introducing an under-ballast mat layer(s) into the track include, but are not limited to: increasing track resilience, reducing ballast breakage, decreasing noise and vibration, and protecting bridge decks. One of the most essential parameters used to evaluate the performance of UBMs is the bedding modulus. Currently, the German Deutsches Institut für Normung (DIN) 45673 is the only test standard specifying procedures to quantify UBM bedding modulus by placing the UBM between two steel plates. However, steel plates might not be an ideal representation of the actual track loading environment. Thus, other types of support conditions have been used to test the UBM bedding modulus, including concrete block and the geometric ballast plate (GBP) specified by European Standard (EN) 16730. How these different support conditions affect the performance of an UBM has not been fully investigated. To better quantify the effects of varying support conditions on UBM bedding modulus, testing was performed in the Research and Innovation Laboratory (RAIL) in University of Illinois at Urbana-Champaign (UIUC). It was found that for a specific type of UBM, the tested bedding modulus values were similar when supported by the steel plate or concrete block, while the value was considerably lower when the mat was supported by the GBP. Finite element simulations were performed to further study the stress distributions under these various support conditions. The results from this study can help practitioners better represent the application environment during the UBM bedding modulus tests by suggesting the appropriate support condition.