Cost and delivery times are key variables used by shippers to determine freight mode choice. Unfortunately, transportation planners wishing to examine truck versus rail trade-offs on major state and regional corridors use models that rarely capture the effects of cargo weight, running speeds, network capacity, or route characteristics, even though they are key inputs to any logistical analysis. Moreover, current models are incapable of fully internalizing external or social costs into their calculations, a failure that becomes more important as sustainable strategies are sought by society. Therefore, in three critical areas of transportation planning—network capacity, route features, and operating characteristics—most existing models are deficient. This study gives planners a mechanistic method to determine variable rail costs on a single corridor. When this model is combined with the latest truck operating cost mechanistic models, the cost differentials that underlie the choice of truck versus rail will be revealed. The model, C-TRIT, is part of a study sponsored by the Texas Department of Transportation to support freight movement on the extensive state network of multimodal corridors.