To remove some of those trucks from the clogged streets and bridges, transportation officials are exploring ways to increase the number of rail cars crossing over to Brooklyn. On Thursday, the Port Authority of New York and New Jersey completed the first phase of that inquiry by releasing a draft study examining the potential environmental impacts of expanding the float bridge or building a tunnel under the harbor.

“Right now, the existing rail barge is a relatively small factor that we expect to grow,” said Patrick J. Foye, the executive director of the Port Authority, which plans to build a new float bridge at Greenville Yard in Jersey City. “It is a very small contributor to taking trucks off the highways,” he said, while adding that “every truck that’s taken off the highway is a positive.”

The cost of the five options for expanding the float bridge now under review range from about $100 million to $600 million, the Port Authority estimates. A four-mile-long, underwater tunnel that could accommodate freight trains, and possibly trucks as well, would cost far more — from $7 billion to as much as $11 billion, none of which has yet been arranged.

“Any type of rail tunnel is going to be a very expensive endeavor,” Mr. Foye said. Most of the money, he said, would most likely have to come from the federal government and private sources, rather than the Port Authority.

The steep cost has not deterred Representative Jerrold L. Nadler, a Democrat from Manhattan, who has been trying to build support for a cross-harbor rail tunnel for two decades. Mr. Nadler welcomed the Port Authority’s draft environmental impact statement as an important step toward his goal, noting that it had found that a tunnel for freight trains would divert 9.7 million tons of cargo from the region’s roadways each year.