This paper presents the conceptual framework for a predictive network equilibrium model of a freight transportation system in which the generation, distribution, modal split and assignment of freight movements are performed simultaneously. A neoclassical profit maximization model is stated for the supply-side of the transportation market, the demand-side is represented by a spatial price equilibrium model, and the economic mechanism which integrates the supply and demand submodels is described. The theoretical limitations imposed on the model by the requirement that it be capable of solving large-scale problems are also addressed.