The Metro subway has been a District-only operation for more than a year, but Virginia joins the system next Friday and Maryland will be next.

Sometime in November, if Metro's present construction schedule holds, another new line will open. It will extend from the present Rhode Island Avenue station through northeast Washington to Silver Spring.

Suddenly, the Metro system will be truly regional. Silver Spring to National Airport in 28 minutes, no traffic. After next Friday, three of the next four Metro line openings will be to terminals in Maryland.

About 60 miles of the planned 100-mile system is virtually certain to be built, including the two extensions to the Beltway. Other lines are less certain. Here is Metro's present construction schedule and the status of the lines:

November 1977 - The present "red line" from Dupont Circle to Rhode Island Avenue is extended to downtown Silver Spring. Four new stations will open - three of them in Northeast Washington. Completion of this segment is assured, although the schedule could slip.

July 1978 - The line which opens from National Airport to Stadium-Armory July 1 of this year is extended across the Anacostia River and out the U.S. 50 corridor to the New Carrollton station. That segment will connect with the Beltway near U.S. Rte. 50 and Md. Rte. 450. Five more stations will open. Completion of this segment appears assured.

June 1979 - A spur line from the new Rosslyn station west through Arlington County opens. Generally, the line follows Wilson Boulevard and Fairfax Drive to Glebe Road. Four new stations will open. Completion appears assured.

November 1979 - A spur line from the Stadium-Armory-New Carrollton line opens. It crosses the Anacostia River and proceeds out Benning Road, East Capitol Street and Central Avenue to its intersection with Addison Road. Three new stations will open. Completion appears assured.

September-October 1980 - The long-suffering citizens along Connecticut Avenue begin to get some payoff at the same time that Alexandria and Fairfax County get their first stations. The Dupont Circle line is extended to Van Ness Centre and the Washington Technical Institute. The National Airport line is extended through Alexandria to Huntington, just south of the Beltway. A total of seven stations will open. Completion appears assured.

July 1981 - The Connecticut Avenue line is extended from Van Ness Centre under Yuma Street NW to Tenley Circle, up Wisconsin Avenue through Friendship Heights to Bethesda, with a terminal at Grosvenor Place, north of the Beltway. Five new stations open. Completion appears assured.

September 1981 - Four sections are scheduled to open but only one seems certain: The Wisconsin Avenue line is extended through Rockville to Shady Grove on the Interstate 270 corridor. Four new stations open.

A line from the Pentagon across the 14th Street Bridge, then up 7th, U and 14th Streets to Columbia Heights is scheduled to open and parts of it - including the bridge and some sections of tunnel - have been constructed. But the line is under study and funding for completion is uncertain. Also scheduled are extensions from the Ballston (Glebe Road) station in Arlington to Vienna in the Interstate 66 corridor and an extension from downtown Alexandria into Fairfax County at Springfield/Franconia. Both of these lines are under study and unfunded.

February 1982 - The Silver Spring line is extended through Wheaton to a Glenmont terminal. The exact location and construction technique for that line are under study, although its ultimate completion in some form appears likely. The line is not funded.

June 1983 - The last links are scheduled to open but both are in dispute. A 7th Street Southwest extension crosses the Anacostia and penetrates through Southeast Washington into Prince George's County. The Columbia Heights line is extended northeast, intersects the Silver Spring line and runs out through Prince George's County to Greenbelt or College Park.