Metro invited the public on a walking tour yesterday of three stations and tunnels still under construction, and drew a mixture of self-acknowledged subway freaks, curious tourists and community residents who wanted to see the results of all the noise and traffic congestion in their neighborhoods.

The three sites - the Benning Road station at 45th Street, Benning Road and Central Avenue NE, the Capitol Heights station in Prince George's County just over the District line at East Capitol Street and Southern Avenue, and the Addison Road Station in Prince George's County at Central Avenue just east of Addison Road - all on the Blue Line extension from the Robert F. Kennedy station are scheduled to open in late 1980.

The 175 people who took the tour listened to guides explain about drainage troughs, backup lighting, cables and fans to help control excessive heat and fires. They poked fingers into grooves in track slabs to feel the rubber-like insulation that helps absorb sound.

They walked through cold, dark, wet tunnels - both box-shaped and circular - and through a wood-framed drainage pumping station. They craned their necks to look at the ceilings of the Benning Road and Capitol Heights stations, which resemble waffle irons that have been folded into an arch, and the open Addison Road station, which has a concrete canopy with a peaked skylight over the center.

And the small crowd said they loved it.

"I watched them tear down the hill (near the Capitol Heights station). I used to play on that hill," said Arthur Miller, 23, who lives in Northeast Washington. He came yesterday, bringing along his year-old daughter Kahlesia - who slept through most of the tour - because he said he wanted to "see what they did to it."

Miller said he purposely has not ridden the subway because he is waiting for the Capitol Heights station to open. "I'm waiting to ride this," he said, jabbing his finger at the station. "This area is improving. It might grow to be something."

One of the guides for the tours was Randy Kuning, who grew up in Montgomery County but lives and works now in Baltimore where he designs radars for Westinghouse.

"Some people know a lot about baseball, I happen to know about Metro," Kuning said. He said he has volunteered his time to give Metro tours since 1973. He has been interested in the subway since the late 1960s, and said he gained his vast knowledge of the system by "poking around in offices, talking to people, and asking for reports."

Robert Cohen of Chevy Chase said he and his son Clark took the tour yesterday because they are "buffs about Metro. We're trying to learn all we can about it."

Clark Cohen, 13, said he often rides the subway. "He doesn't ride it because he has to. He rides it because he likes it," his father said. "He likes the design of contemporary architecture."

Joe Aqua, 26, of Oxon Hill, attended both the 10 a.m. and noon tours. "I've been interested in the Metro project since high school. I've always been fascinated by trains and subways."