The project would lie in the borough, which, along with the railroad, is listed as an intervenor in the case, in support of the board. The state of Alaska also opposed the conservationists' motion.
The Surface Transportation Board last November gave final approval to the railroad to build and operate about 35 miles of new rail line. The proposed line would run from Port MacKenzie to near Houston.
The board, in announcing the decision, said the proposed line would provide rail freight services between the port and Alaska's interior and said it also would support the port's development as an export-import facility for intermodal and bulk-material resources.
Last month, the U.S. Army Corps of Engineers issued a permit allowing the railroad to fill in nearly 96 acres of wetlands for the line.
Opponents, in a news release, said the project would open the door for shipping Alaska coal overseas, but they say that would come at the expense of local families, landowners and salmon fisheries.
"We already have three tidewater ports in south-central Alaska and we don't need to waste public money on another," Bob Shavelson of Cook Inletkeeper said in the release.
Sullivan said the port "will be South-central's future industrial center." She said the project will create thousands of jobs and allow for development of more minerals in the region.