Metro ran the first excursion of its short-cut line from Southwest Washington to the Pentagon yesterday, but it had to -e done on foot.

Anybody who wants to ride a train on the Yellow Line through a tube under the Washington Channel and across a new bridge over the Potomac River alongside the 14th Street highway spans will have to wait until early 1983.

Eventually, the new crossing will be part of a line that is planned to run from Springfield, Va., through Alexandria and Washington to Greenbelt, Md. But when it opens in 1982, it will provide only a short shuttle service between National Airport and Gallery Place station at 7th and G Streets NW.

The shuttle service will avert the need for roundabout and crowded travel for many commuters via Rosslyn, Va.

Although the new Yellow Line crossing was an afterthought in Metro planning, about 40 people who took yesterday's first walk - mostly suburban Virginia and Metro officials and their families - agreed the line would be invaluable now.

"I'd like to see it operating tomorrow," said Joseph Alexander, a Fairfax County supervisor and Metro director. "But I'm glad we persevered [in insisting it be built]."

Standing on the bridge, just short of the Virginia shore, Rep. Herbert E. Harris (d-Va.), a former Metro board chairman, turned to Alexander and recalled the birth of the Yellow Line.

"Joe," he said, "remember when we pulled that yellow pad in front of us and drew the line [connecting the Pentagon with L'Enfant Plaza]."

"Sure do," Alexander replied.

Yesterday was the first walk-through of a completed Metro subway structure for Richard S. Page, Metro's new general manager. He was accompanied by his wife, Edith and their two children. Son Stephen, 14, wore a T-shirt picturing a train and emblazoned "Metro man."

The new crossing includes a sunken steel tube, resembling a giant oval pipe 35 feet across. The pipe was fabricated in three 340-foot-long sections at a shipyard at Port Deposit, Md., and floated to Washington, pulled by tugboats.

These sections of tube were sunk into a trench dug in the Washington Channel bed between Maine Avenue and East Potomac Park (Hains Point). Rails will be installed on concrete trackbeds that already have been poured inside the tube.

Construction of the line was slowed in August 1977 by flooding that curtailed train operations on the existing Blue Line for four days.

Water leaked through a cofferdam that had been erected to seal off the end of the Seventh Street subway at Maine Avenue from the segment where the tubes were waiting to be sunk under the channel. The water poured beneath Seventh Street and, at the L'Enfant Plaza station, poured onto the Blue Line tracks on the lower level.

Some seepage of water into the subway was observed yesterday beneath Main Avenue. Larry Purnell, Metro construction engineer, said cement grout is being pumped into the subway's concrete walls to halt it.

Page said three other segments of subway are scheduled to open before service starts on the Yellow Line river crossing in 1982.

The next Metro section to be opened, late this year, will carry Orange Line trains between Rosslyn and the Ballston station in Arlington, Page said. This will be followed in December 1980 by a Blue Line extension from the Stadium-Armory station via Benning Road and East Capitol Street to the Addison Road station in Prince George's County, and in fall 1981 by a Red Line extension out Connecticut Avenue from Dupont Circle to Van Ness station in Northwest Washington.

Page said that both the Van Ness and and new river crossing routes depend on the delivery of some of the 90 new cars ordered to augment Metro's current fleet of 300. CAPTION: Picture 1, Rep. Herbert Harris and others finish walking tour of Metro train tube under the Washington Channel.; Picture 2, By 1982 Metro trains in this tunnel will provide D.C. to Pentagon service along the Yellow Line. Photos by Ellsworth Davis - The Washington Post; Map, No Caption, The Washington Post