The regional task force that is studying whether it makes sense to complete the 100-mile Metro subway as planned voted yesterday to change the route through Anacostia and to reject a Greenbelt route vigorously backed by the Maryland Department of Transportation.

Members of the task force also took loud exception to a new letter from the federal government that appears to require even more study and delay before Metro can start building beyond the 60 miles that presently are financed.

The task force, composed of politicians and officials from the Metropolitan Washington Council of Governments, Metro and the regional Transportation Planning Board, is nearing the final stages of its months-long analysis of four uncompleted suburban Metro lines. Yesterday, the group was scheduled to select three possible Metro systems for the area beyond 60 miles.

The task force did not get that far because the Northern Virginia delegation simply had not reached a consensus on what options it wants to leave in the study. Two Virginia lines - between Alexandria and Franconia County and between western Arlington County and either Vienna or Tysons Corner - are candidates for truncation or elimination.

The District of Columbia and Prince George's County came with a unified position on two lines they share. One of them runs from downtown Washington through the new south-west to Anacostia and into southern Prince George's County near Branch Avenue and the Beltway. That route was reaffirmed yesterday but redrawn on the map to include a stop at St. Elizabeths Hospital. Dropped from consideration was a route that would terminate south of the Beltway at Rosecroft Raceway.

The other line runs north from downtown Washington up 7th, U and 14th Streets to Columbia Heights, at about 14th and Kenyon Streets NW.

From there it is scheduled to head through upper northeast Washington and cross the existing Red Line at Fort Totten - one of the stations that will open for service Feb. 6. From Fort Toten the line will go through West Hyattville and Prince George's Plaza, then east and north to College Park and Greenbelt.

The action by the task force yesterday left two options on that Greenbelt line still under study. Under the first option, the line would be truncated at Columbia Heights, and never penetrate Prince George's County. Under the second option, the line would be completed as planned.

Thrown out in that process was another alignment that would have gone almost due north from Prince George's Plaza and terminated at the Beltway's intersection with interstate 95.

The Maryland Department of Transportation had backed the route because of its convenient access to 1-95, among other reasons. The county, under strong pressure from the neighborhood next to the I-95 terminal, had resisted the state.

"It is time to stop further study on this alternative," Prince George's County Council member Francis Francois said. "It is not going to happen."

The task force hopes to complete its work by the end of February and then send to the federal Department of Transportation a report containing financial and cost-effective analysis on the various options still in the study.

Russell Scoville, the man who monitors Metro programs for the federal Urban Mass Transportation Administration, an arm of federal DOT, appeared at the task force meeting yesterday bearing a letter from UMTA chief Richard S. Page.

In that letter, Page outlined a series of steps UMTA was prepared to take when it receives the final task force report, but also laid down some conditions that appeared to be new.

The one that drew everyone's fury to Scoville the bearer of the tidings, was the statement that "for UMTA review," the task force study must contain cost and ridership comparisons between 63.9-mile system and any of the alternatives.

Sixty miles are funded; another 3.9 miles, Scoville explained, are not subject to federal review, But that 63.9 mile total does not include the line from Silver Spring to Glenmont, and all the studies done by task would be constructed - with the early agreement of UMTA.

Therefore, the comparisons UMTA says must be included are not part of the vast material on costs and ridership that have been developed during the study. The study would have to be extended for months, it was estimated, to develop the data UMTA requested.

Metro board chairman Joseph Wholey characterized the paragraph as a trap and suggested the task force ignore it.

Francois, the chairman of the task force, told Scoville that if the federal government wanted the Glenmont numbers included it had the power to order that 16 months ago, when the study started.

"There will come a time," Scoville said, when UMTA's Page and Transportation Secretaty Brock Adams must decide how much of Metro they will support. "The difficulty we have," he said, "is that we do not know what the . . . cost effectiveness of the Glenmont route is."

Page, in a telephone interview, said a new study was not what UMTA has in mind and that the call for comparisons reaffirmed anearlier instruction from Adams to Metro officials. "I would be amazed if that kind of data is not available to local staffs," he said.