Metro, for the first time since subway service began in 1976, will run shortened two-car trains as an economy measure on all three of its rail lines on weekends, starting today.

Until now, all regularly scheduled Metro trains outside of weekday rush hours have had at least four cars, and frequently six. The longest trains, eight cars, usually run only during rush hours. Each two-car unit has about 160 seats.

Cody Pfanstiehl, Metro's chief spokesman, said an exception to the two-car service will be made Sunday afternoon, when longer trains will be operated to carry spectators to and from the football game between the Washington Redskins and the San Francisco 49ers at RFK Stadium.

Pfanstiehl said the decision to cut other weekend trains to two cars resulted from budget cuts. He said they were ordered by the transit authority's board of directors, which represents the eight local governments that are partners in the system and subsidize its operating losses.

While overcrowding may occur at some hours, Pfanstiehl said the reduced train lengths are expected to save about $600,000 a year in electric power costs, payrolls and car maintenance. Longer trains are scheduled to be restored on weekends next April at the start of the spring tourist season.

Pfanstiehl said the reduction to two-car trains could create operating problems for the rail system. If the electric motors in a two-car train break down, he said, the entire train will stall and block the line until another train pushes or pulls it to a maintenance yard. If the motors in part of a longer train break down, however, the train can be moved with power supplied by the still-operational motors in other cars.

Metro cars are hooked together into two-car units, with an operator's cab at each end of the unit. The cars cannot be operated singly.

The cut to two-car trains on weekends is part of a broader Metro policy of running shorter trains.

Even during rush hours, some trains on the Blue/Orange line between Ballston station in Arlington and Addison Road in Prince George's County have been reduced to four cars, creating severe overcrowding and leaving passengers waiting on platforms, sometimes causing them to miss bus connections. Pfanstiehl said he received six complaints on Thursday alone about crowding on that line.