A growing imbalance between demand for freight mobility (in terms of ton and tkm) and the supply of transport (in terms of lanes of infrastructure) in and around seaports is jeopardising the sustainability of its future development. Given the ever growing globalisation of the maritime industry any shortage in (trans-port) capacity is negative. In addition to cost-effective pricing and management application of technological improvements in the field of transport and transhipment will make port development more sustainable. New (underground) modes of transport could help, the more because transport capacity extensions on the earth surface are more and more affected by limitations in speed and size as a function of density of population and the existence of biotopes, especially around (port)towns. Moreover improved surface transport systems such as lash, pushing convoys and double-stack trains are not applicable everywhere in the BNL. Of course expansion of trans-port capacity can be realised in various ways: by using bigger vehicles, by re-organising transportation spatially and timely, etc., but by creating new \u93ways\u94 or modes of transportation a much greater extension in capacity will be achieved. The feasibility of any new transport mode however depends upon cost recovering. An in-depth study of the situation in Antwerp and Rotterdam has been performed in order to acquire correct understanding of technical, economical and commercial consequences. It includes travel time and reliability analyses and a cost analysis. UCM (Underground Container Movers) as a pilot project demonstrates that \u93Underground Tubular Transport\u94 contains interesting features to avoid congestion and to increase the efficiency of terminal operations in seaports confronted with congestion due to constant growing truck volumes. The volume to be transported through the UCM-system must be high enough to cover all costs (more than 2 million TEU in the business case). Application in due time of the UCM-system in the port of Antwerp might also alleviate the congestion on certain hinterland connections, especially on high ways, which together with lifecycle considerations increases the degree of sustainability of this new way of transportation