The Massachusetts Bay Transportation Authority's (MBTA) plan to extend the line cleared an important hurdle last month when state Environmental Affairs Secretary Robert Durand approved plans to run the trains through the sensitive conservation area.

But now the agency and other supporters of the rail line face a problem on another environmental front: the Assonet Cedar Swamp Wildlife Sanctuary in Lakeville.

The 750-acre Massachusetts Audubon Society conservation area, which is a state designated habitat for endangered species, is in the path of the railroad's planned eastern branch between Taunton and New Bedford.

The low evergreen wilderness area is home to the rare Hessel's hairstreak, a butterfly that comes from a caterpillar that feeds only on Atlantic white cedar trees, which are found in the swamp.

Officials and residents in Lakeville and the Massachusetts Audubon Society have called on the MBTA to provide assurances the swamp will be protected when the railroad is built through the area.

"We have some significant concerns," said Heidi Roddis, senior environmental policy specialist for Massachusetts Audubon. "It has been very difficult to determine what the impact would be because the MBTA has not provided many details."

Stephen A. Olivier, a Lakeville resident whose property is next to the swamp, said the MBTA made little mention of the area in the environmental impact report on the line. "There are a lot of rare species there," he said. "It is a nesting area for great blue herons.

Olivier is a leader of the newly formed Friends of the Assonet River Environment, which will hold a public meeting at 7 p.m., Sept. 26, at Apponequet Regional High School in Lakeville.

The Lakeville Board of Selectmen has voiced concern about the swamp and has urged that safeguards be implemented. The selectmen also want assurances that the trains will not disrupt the new Freetown-Lakeville Regional Middle School, which is near the planned line.

Unlike Easton and Raynham activists, who are trying to block the rail line from coming through the Hockomock Swamp, defenders of the Lakeville conservation land, at this point, are only calling for assurances that the environment will be protected when the railroad is built.

"Our primary concern isn't blocking the railroad so much as making sure our questions are answered," said Lakeville Selectman Chawner Hurd.

Jon Carlisle, a spokesman for the state Office of Transportation, which oversees the MBTA, said drainage systems will be installed and other steps will be taken to protect the swamp.

Carlisle noted that a freight line already runs through the swamp. When the commuter rail tracks are installed, environmental improvements will be added to the freight line, he said.

"We think the introduction of a commuter rail could make the area even better protected," Carlisle said.

With Secretary Durand's approval of the MBTA's environmental impact report, the regulatory process shifts to federal, state and local agencies that issue environmental permits for the different construction phases.

Roddis, of Massachusetts Audubon, said the organization will make its case for protection of the Lakeville swamp and the similar Acushnet Cedar Swamp in New Bedford when the MBTA applies for permits.

The Assonet Cedar Swamp is considered a wilderness. Massachusetts Audubon has not developed trails or built a nature center on the property. It is accessible mainly by canoe on two streams that run through it.

Cedar swamps are rare in Massachusetts and provide a special environment for plants and animals, according to Roddis.