This dissertation presents the development of a computer system, wich performs the assignment of multicommodity freight transportation flows on a multimodal network. This systems should be used as a tool in strategic transportation planning. Initially the software the has been conceived to search for the system`s optimum, by minimizing the total transportation cost in the network. The Dissertation proposes an adaptation to make possible for the system to calculate the user`s optimum, by minimizing the costs of the individual paths of the transportation network users. To Implement the multicommodity flow assignment on the multimodal network, the software makes use of na adaptation of the Frank and Wolfe algorithm to solve the multimodal, multicommodity problem.