Abstract: Estimation of fatigue cycles for railroad bridges is complicated by the variation in live-load moment as a train passes across a bridge. Railroad trains create a maximum bending moment cycle like highway vehicles but also generate other potential cycles owing to the passage of the railcars over a bridge. The magnitude of these other cycles depends on other factors besides weight. Fatigue design of railroad bridges in North America is based on the expected number of cycles for a typical unit-coal train. While the assumed train and its frequency of application of loading are adequate for design of new spans, it does not represent all loadings. This can result in error when rating for and estimating fatigue life. The current study displays the fundamental formulation for moment range, the variation in live-load moment, for railroad loadings. Train types are displayed representing different eras of weights and car types as these have changed over time. Additionally, information is provided concerning span response and impact.