Metro Board members tentatively agreed yesterday to pull 16 cars out of peak-hour service on the subway system in January as a test of the effects of a possible early opening of the Yellow Line between Gallery Place and National Airport.

The test, scheduled to begin Jan. 3, could produce some peak-hour crowding. If it is successful, however, it may encourage Metro officials to move up by several months the Yellow Line opening, which would cut nearly 15 minutes off a trip between National Airport and Capitol Hill.

The board is scheduled to decide by March 10 on the actual opening date for the Yellow Line. Board member Cleatus Barnett of Montgomery County, citing problems that followed the opening last year of Red Line service to Van Ness, voiced concern that the system's rail-car supply might once again be spread too thin.

He insisted that the test simulating the diversion of cars to the Yellow Line precede announcement of any Yellow Line opening.

Metro recently has been more successful building tunnels and track than finding rail cars. Strikes and other manufacturing problems have plagued the rail-car supply almost from the system's opening six years ago.

Metro is currently awaiting delivery of rail cars from Italy, the first 36 of which had been earmarked for the Yellow Line, where stations and track have been completed to Huntington for some time.

While Metro staff members says service to Huntington must await arrival of the Italian cars, they believe service on operating lines has has improved enough to transfer some existing cars to make the Gallery Place to National Airport shuttle a possibility as early as next April.