One of the last large pieces of vacant land in Alexandria will be developed by a Richmond-based railroad and a company owned mostly by a Netherlands firm as a major office, hotel, retail and residential complex, the land's owners said yesterday.

The Richmond, Fredericksburg and Potomac Railroad Co. and Savage/Fogarty Companies Inc. announced they have entered into a joint venture to develop the 38-acre site south of National Airport between the George Washington Memorial Parkway and the Potomac railroad yard.

How the site will be developed has been a matter of debate for in Alexandria for a dozen years and Mayor James P. Moran said the announcement will not end the debate.

"I anticipate a lot of controversy," he said. "The real struggle will be to balance the benefits of development with the demands on the road system and environment."

Jeffrey H. Riddel, Savage/Fogarty vice president, said it was too soon to give an estimated cost for the project, but he said construction could begin as early as fall of 1987.

"We recognize that the project will have a major impact on Alexandria and we will actively seek the involvement of the city in the planning and design process. We will also be sensitive to their concerns," RF&P President Richard Beadles said in a statement.

Beadles and Julien Redele, chairman of Savage/Fogarty's executive committee, said the project's impact on transportation and the environment would be key issues. The Richmond railroad is the largest private landholder in Alexandria and it told the city officials last month that it was on the verge of developing the 38-acre tract.

Savage/Fogarty Companies, owned by a Netherlands-based corporation, has developed a number of office complexes in Alexandria, including the $125 million TransPotomac Canal Center on the Potomac waterfront and the $60 million Braddock Place near the Braddock Road Metro station.

"I know the developers are prepared to work with the City Council and the citizens to the optimum degree," Moran said last night.

"My two major concerns are to preserve the scenic beauty of GW Parkway and to find a way to deal with the traffic, which comes close to a gridlock in that area," the mayor said.

The latest project will bring "probably thousands of jobs" as well as a significant amount of property tax revenue to Alexandria, Moran said. City officials have said that development of the tract is inevitable.