Trailways Inc. may close its downtown terminal and move to a new facility to be built behind Union Station on land now owned by the Redevelopment Land Agency, under a plan announced yesterday by city officials.

However, a spokesman for the Dallas-based bus company expressed surprise yesterday at the announcement from the office of Mayor Marion Barry, calling it "a bit premature." Roger Rydell, Trailways' director of public relations, said that while the firm is interested in the RLA site, it is also considering several other locations in the District.

If the Trailways deal is successfully negotiated, the bus firm would be joining its competitor, Greyhound Lines, in moving from 12th Street and New York Avenue NW to the area near Union Station. Greyhound, whose terminal is across the street from Trailways, announced plans last June to move to North Capitol and K streets NE. That site is a block from the RLA site that would house the new Trailways terminal.

The city released a statement by Robert L. Moore, D.C. Housing and Community Development director, announcing that the RLA had given him authority "to negotiate a land agreement for a new Trailways bus terminal."

Moore's statement said he will negotiate the sale price of the 71,000-square-foot site with McLean-based Watergate Development Inc. He said Watergate would build the new terminal and in exchange would receive part of the old 12th Street Trailways site for development. That site, near Metro Center and the city convention center, is considered extremely valuable.

The Greyhound site across the street was estimated to be worth between $6.5 million and $19 million, depending on its use.

Moore's statement said Watergate had already lined up $9 million in loans to pay for buying the RLA land and building the terminal. Neither he nor Watergate officials could be reached for comment yesterday. Rydell, however, described the plan as "very, very much a question-mark" and said its success depended on the land cost and numerous other factors.