This paper presents a mathematical programming-based model for evaluating the impact on a mixed railway network from proposals for infrastructure improvement and capacity expansion that are oriented mainly toward increasing freight transportation. The authors have applied this model to extend elements of an existing mixed railway network, perform relatively less costly actions on the network, and to enhance capacity by adding new blocking/control systems at specific locations. These aspects are usually not taken into account in models for regional planning. Rather than a model whose sole focus is on railway capacity expansion, our approach combines capacity-expansion with network design. Because the way investments generate returns to the freight transportation system is of utmost relevance for these types of problems, this model is based on the efficient frontier between investment and operating costs. The model is tested on a regional network in the Mediterranean Corridor.