Metro, the Washington area's transit system, will begin Sunday and holiday subway service starting tomorrow, giving shoppers, tourists, churchgoers and Redskins rooters a new traffic-free way to get around town.

Trains will run at 10-minute intervals from 10 a.m. to 6 p.m. Extra trains will be operated on the Blue/Orange Line to handle crowds leaving the Stadium-Armory station after the Redskins game with the Houston Oilers.

The regular off-peak fare of 50 cents will be charged for all rides, regardless of distance. Senior citizens will pay 25 cents.

When the first short section of the Red Line was opened in 1976, trains were run Monday through Friday. After the line was extended and the Blue Line opened the following year, Saturday service was added.

The new expansion lifts the system from being chiefly a commuter railway into a full-service transit system.

To mark the start of Sunday service, six downtown churches will join in a 15-minute ceremony at 12:30 p.m. in the 13th and G streets mezzanine of the Metro Center station. The service will be held by the Rev. Stephan Kingshofer, assistant rector of the Episcopal Church of the Epiphany, which is half a block from the station entry.

At the other end of the same station, Woodward and Lothrop will open the doors of its downtown flagship store for regular Sunday operation for the first time in nearly a decade.

Working through the retail bureau of the Metropolitan Washington Board of Trade, Woodies lobbied for Sunday train service and agreed to open the store if it were provided. Its suburban stores already are open on Sundays.

As a one-time promotion, Woodies will give a 50-cent subway Farecard for the ride home to customers who make purchases of $1 or more tomorrow.

Woodies biggest downtown rival, Hecht's, already is open on Sundays. Hecht's is a block from the Gallery Place subway entry at 7th and G streets NW.

Leonard Kolodny, manager of the Board of trade retail bureau, said most smaller downtown stores have adopted a wait-and-see attitude, with many likely to open in the future if the train service creates a significant pedestrain traffic.

Metro's financial planning chief, William I. Herman, said he expects about 35,000 people to ride the trains on a normal Sunday. That figure will be increased by an uncertain amount on the seven Sundays this season on which the Redskins play home games.

The projected ridership compares with about 70,000 on Saturdays and 260,000 currently on weekdays.

The Robert F. Kennedy Stadium, where the football game will be played, is a two-block walk from the north exit of the Stadium-Armory subway station at 19th and East Capitol streets.

The station is on the Blue/Orange Line, which extends in one direction to the New Carrollton station in Price George's County and in the other to Rosslyn, the Pentagon and National Airport. Passengers using the Red Line from Silver Spring may transer to stadium-bound trains (marked "New Carrollton") at Metro Center.

Parking at Metro owned lots and the Montgomery County garage at Silver Spring will be free on Sundays and holidays. Lots on the Red Line are located at Takoma, Fort Totten and Rhode Island Avenue stations. Lots on the Blue/Orange Line are located at the Minnesota Aveneu, Deanwood, Cheverly, Landover and New Carrollton stations.

All regular Metrobus routes will remain unchanged. Buses that run from outlying points to downtown Washington will go all the way downtown, and will not be turned back at subway stations as they are on weekdays.

Metro also will continue running 13 special bus routes to and from the football game.

Train service on Monday, Labor Day, will be operated on the new Sunday schedule.

In a related development, Metro will begin running a new experimental bus lines today from 10th Street and Pennsylvania Avenue NW to the tip of Hains Point in East Potomac Park. The route, marked "HP-2," will run every half hour from 10 a.m. to 8 p.m. every day of the week until Oct. 28. It is subsidized by the National Park Service to provide access to recreation areas.