One of the assumptions generally made in hub location
problems is that the interhub network is a complete graph, but
the spoke nodes are not always interconnected. Direct shipment
between spoke pairs is also not allowed and the flow of cargo tra-
1 Twenty-foot Equivalent Unit. verses at most two hubs. In practice, especially on an internationalscale, such a rigid structure is unlikely. Direct shipment by trucks is
commonly used, and complete multimodal networks are not feasible.
These facts bring many modeling and methodological challenges
which need more investigation. A considerable part of the
literature has a direct shipment option in their model. However,
only Rodríguez-Martín and Salazar-González (2008) and Alumur,
Kara, and Karasan (2012a) assume an incomplete underlying
network.