Concrete crosstie usage in North America continues to increase for rail transit and heavy axle load freight railroad applications. As such, it is important to design optimized crossties to save both capital and maintenance funds. Recently, a method for quantifying concrete crosstie bending moments using concrete surface strain gauges has been developed, deployed, and validated. Data from this method are used in this paper for (1) building a model to quantify sources of variability for field bending moments and the relative influence of each source, (2) generating an accurate model to predict bending moments at the two field locations surveyed, and (3) comparing the relative effects of predictor variables on rail transit and heavy axle load freight rail modes to determine their influence on service bending moments. Results show that it is possible to develop a reliable model to predict bending moments, and that several factors have a strong influence on these predictions, namely vertical load, temperature gradient, and axle location within a railcar truck. The most significant factor is the crosstie support condition, especially with respect to center moments. While the aforementioned model’s primary utility is for the two sites and railroad systems surveyed, the model provides a valuable tool for determining which variables are the most critical for inclusion in the future mechanistic design of concrete crossties.