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They are roll-on/roll-off vessels that can carry vehicles.

The two sides are fighting over a six-year master contract that covers about 25,000 port workers employed in container and roll-on/roll-off operations, according to the US Maritime Alliance, known as USMX, which represents shipping firms, port associations and marine terminal operators.

Shippers might opt to go with them because they can handle roll-on/roll-off ships — as Baltimore could — which are designed to carry cars, trucks, buses, trailers and other vehicles.

Automobiles are transported in vessels known as roll-on, roll-off ships.

Ports in Charleston, Jacksonville, Fla., Newark and Norfolk, Va., can also handle roll-on, roll-off ships.