The committee overseeing possible cutbacks of Washington's Metrorail system agreed yesterday to consider substituting so-called light-rail trolley cars on two routes in Washington and Prince George's County.

In a major victory for the Montgomery County government, the committee voted to drop its study of alternatives to the Silver Spring Glenmont subway - an action that makes it more likely that the line actually will be built along its proposed alignment beneath Georgia Avenue.

Both actions were taken by a joint policy committee created by the Metropolitan Washington Council of Governments (COG), the COG transportation planning board and the Metro transit authority.

The committee was created to direct an analysis of alternatives to completing all 100 miles of the projected regional rapid transit system. The analysis is required by Congress and the U.S. Department of Transportation as a possible way of cutting construction costs.

Until yesterday's meeting, the only alternative being considered to Metro's full heavy-duty rail system was the introduction of express bus service on some routes.

The new proposal - made jointly by the District of Columbia and Prince George's County governments - would introduce, for the first time in this region, modern trolley cars on two routes.

One route wold be along 14th Street NW in Washington, to an outer terminal at Missouri Avenue, just south of Walter Reed Army Hospital. Although details were not spelled out, the line apparently would terminate at the Gallery Place station, 7th and G Streets NW.

The other route would occupy an electric power line right-of-way owned by the Potomac Electric Power Co. in Prince George's, running from the Ft. Totten Metrorail station to a terminal near the Capital Beltway.

If built, both would substitute for segments of Metro's Greenbalt route, which has stirred controversy both in the city and in the suburban county.

Under the proposal, the suburban trolley line would be supplemented by increased commuter service on the parallel Baltimore & Ohio Railroad line, which already has a few rush-hour trains serving Hyattsville, College Park and Laurel.

Light rail - the description apples to its passenger-carrying capacity, less than that of a subway - enjoyed a recent vogue, notably in a few American cities where remaining old streetcar lines are being upgraded. Only in Buffalo, N.Y., has a plan for a totally new light rail system been officially approved.

Typically, a light rail line might interlude some lengthy segments of right-of-way or subway where the cars can operate swiftly. On the other segments, however, the cars might be mixed with street traffic. The latter description would fit 14th Street, where one of Washington's last streetcar lines ran until 1962.

Light rail's chief attractions are lower track construction costs and possibly fare collection aboard the car, eliminating the need for station attendants.

The request to exempt the 4 1/2 mile Silver Spring Glenmont subway from the cutback analysis was made by the Montgomery County government.

At present, the line is being built from Washington to Silver Spring, and service is schelduled to start next November. However, the Silver Spring station is regarded as an unsuitable permanent terminal and construction is needed to some point beyond.