The Richmond, Fredericksburg and Potomac Railroad has filed suit in federal court, arguing that the National Park Service is unfairly trying to restrict its development rights to a site being considered for an $821 million Navy office complex.

The Park Service contends that the 35-acre parcel in Arlington just north of Four Mile Run and west of the George Washington Memorial Parkway is restricted to use as a railway yard and cannot be developed. Some familiar with the suit said the legal action, which seeks to clarify a 52-year-old agreement between RF&P and the government, could chill the Navy's interest in the site.

The Navy is considering six sites in Arlington and Alexandria for a 2-million-square-foot complex that would bring together employees from leased spaces scattered around the Washington area.

"Clearly it's in our interest to have any title issues resolved, and that certainly had a lot to do with the timeliness of asking a court" to rule on the railroad's development rights, said T. Eugene Smith, a member of Alexandria 2020, a group that oversees the long-range development of RF&P's entire 311-acre rail yard.

C.A. Hartz Jr., an attorney for RF&P in Richmond, said the property represented so much potential that the suit filed Friday in U.S. District Court in Alexandria would have been brought regardless of the Navy project.

"The property is well located," said Hartz. "And given what the demand has been for commercial space and office space in the D.C. area in the past, there is a potential there, there's no doubt about it."

Sources close to the negotiations said the Park Service is trying to hold the Arlington land hostage, trying to force the railroad into giving up its right to a George Washington Parkway interchange that would provide access to Potomac Greens, a much embattled RF&P project proposal in north Alexandria.

"It's leverage to stop the interchange," said Alexandria Mayor James P. Moran Jr., adding that the City Council has "used every means at our disposal to prevent" the development of Potomac Greens just south of the proposed Navy site.

Citizen groups and Alexandria officials have opposed the proposed $500 million Potomac Greens project, arguing that the 38-acre mix of commercial and residential space would cause overwhelming traffic tie-ups on the parkway.

Though the Park Service agreed in the early 1970s to allow RF&P to build an interchange at the parkway where Potomac Greens is now proposed, Congress ordered that the service first conduct an environmental study to assess the impact of the development.

That study, due in a few weeks, will be returned to Congress before the fate of the interchange is decided.

Jack Benjamin, of the National Park Service, said yesterday that he recently met twice with Smith, of Alexandria 2020, and explored an offer whereby the service would allow partial development of the Navy site if the railroad gave up its plans for an interchange.

At a third meeting, Benjamin said talks were cut short because the railroad had decided to file suit.

"Given that RF&P was extremely interested in the {Navy} development, we assumed they would be very interested in working something out," Benjamin said.