Container ships have consistently increased in size and draft, requiring 50-foot water depths. Each large vessel carries more units of cargo at a lower cost per unit, producing economies of scale. Shipping companies have formed alliances to trade and share space on the large vessels. The ships travel fully loaded and operate at peak efficiency. The marine terminal, the next step in the cargo chain, is where high-speed cranes unload the ship, stack the inbound containers and reload the ship with outbound containers.
Large forklifts move the containers onto railcars or trucks bound for inland destinations.
Terminal yards have only so much storage space. Increasing the capacity of marine terminals reduces loading delays and the container ship operates at peak performance. The final link in the intermodal system is the railroad. Rail lines located close to terminal yards, but not constraining storage capacity, allow for the greatest cost effectiveness. Containers can be double stacked on many rail lines for the most efficient form of cargo transport.