This study derives explicit analytical expressions for comparing contact patch dimensions and Kalker’s coefficients for a wheel moving on a roller and compares the results with a tangent track arrangement. The expressions suggest that full size roller rig will underpredict Kalker’s traction coefficients (creepage forces per unit creepage) by a factor that depends on the roller radius. Studying rail-wheel contact mechanics and dynamics in the field conditions can prove to be challenging due to the difficulties in adequately controlling the test conditions that can significantly affect the results, such as track irregularities, rail surface condition, etc. Roller rigs can prove to be a useful tool for such studies. One, however, must be careful when interpreting roller rig test results because of the differences in wheel-rail contact mechanics and dynamics between the track and the roller. The findings of this study, which are consistent with other studies’ conclusions, will allow researchers to relate results with field testing.