A relevant issue for the strategic modeling of interregional freight transport networks is to include external costs as part of a policy that supports the mechanisms for managing and pricing to achieve the social optimum. In this paper, a freight transport model, including external cost, is developed and applied to the Colombian intercity intermodal strategic network involving equilibrium between the phases of distribution and traffic assignment. Each link on the network includes internal costs: time and operation, and external costs: congestion, accidents, air pollution and CO 2 emissions. Marginal costs on the freight transport network are calculated using two approaches. First, it is assumed that an additional unit of demand does not affect the equilibrium of the transport network, and then the marginal cost is estimated as the sum of marginal costs on the shortest path links. The second approach assumes that an additional unit of demand changes the network equilibrium and, consequently, the marginal costs are estimated by calculating the difference between the two equilibrium scenarios. Both approaches are applied to seven selected route corridors covering the most important national freight transport corridors. It was found that both methods produce similar results. Average external costs were rated equal to 0.014 US$/ton-km for highways, 0.000105 US$/ton-km for inland waterway transport and 0.0016 US$/ton-km for rail. In highways, external costs are equivalent to 37% of internal costs, in railways 12%, and in inland waterways they represent only 1%.
