“It’s going to be a heavy lift for funding, but it’s one that I think is doable,” Mr. Nadler said in an interview on Wednesday. He had assembled, he said, a coalition of supporters that included his fellow Democrats, Senator Charles E. Schumer and Mayor Bill de Blasio, all of whom recognized that the federal government needed to invest more in how freight travels because “there’s not much room for additional tractor-trailers.”

Until a rail tunnel is built, New York is left with its float bridge, which Mr. Nadler said was “the epitome of 19th-century technology” before the Port Authority and the city’s Economic Development Corporation started improving it.

On the Brooklyn side of the harbor, the city has installed new links on the waterfront at 65th and 51st Streets to connect the barges with the tracks of the New York & Atlantic Railway, which runs freight trains on the Long Island Rail Road’s tracks. That is why the boxcar full of potatoes was accompanied on the rail barge on Wednesday morning by three cars stacked high with lumber bound for Home Depot stores on Long Island.

The barges, which are operated by the Port Authority’s New York New Jersey Rail subsidiary, carry an eclectic mix of cargo. They have delivered more than two dozen locomotives to the New York City subway system and have carried 190-foot steel girders for the new Willis Avenue Bridge spanning the Harlem River, said Donald B. Hutton, who runs the float bridge.

Mr. Hutton lines up the customers for the service, which he said runs two barges each way every weekday, but could add a third if demand were to increase. On Wednesday, several of the 14 rail cars loaded onto one of the barges on the New Jersey side were empty, on their way back to Sims Recycling’s yard in Sunset Park to be filled with scrap metal.