Metrorail reopened its last closed stations yesterday afternoon, 72 hours after Friday's storm began shutting the system down segment by segment, and officials acknowledged that the storm had caused a new icing problem Metro simply wasn't prepared for.

It was the second time in its seven years of operation that Metrorail, once touted as virtually weatherproof, was closed by a storm. Metro technicians are now studying equipment and design changes to counter problems revealed by Friday's storm, officials said.

What kept the surface lines closed was not snow on the main tracks, but ice on the 750-volt "third rail" that provides power to trains. That ice broke the electrical contact between the rail and the cars' "shoes" that ride along the third rail and draw power. Without that electrical contact, the trains cannot run.

Metro is one of the few transit systems with a cover over its third rails. It was meant to ward off light snow and sleet and prevent accidental contact and electrocution, which officials say it does.

But after Friday's storm, they learned that the cavity between cover and rail can collect ice that can only be removed by hand.

"I don't think we appreciated how severe that problem was going to be until we got this snow," said David Cooksey, Metro's assistant director of general maintenance. "We do now."

The first storm to close Metrorail was the George Washington's Birthday snowstorm of 1979, disproving promises made by designers in the 1960s.

Officials used to boast that "when the automobiles are sliding around on the sleet, the trains will be running swiftly," recalls former public affairs director Cody Pfanstiehl. After the 1979 storm, officials were more cautious in their statements, he said.

The problem in 1979 was snowdrifts on main tracks, station platforms and parking lots, which Metro had little equipment to remove. After the storm, Metro spent about $1 million on plows, blowers and other removal equipment, but paid little special attention to third-rail icing, which had not been a major problem because of a quick thaw.

One special plow that Metro had designed after that storm apparently helped worsen the third rail problems this weekend. "It was designed to toss the snow over the third rail," said rail service director Joe Sheard. "Apparently it tossed it more at the third rail."

On Saturday, plowing and clearing progressed briskly, leading officials to predict close to full service by Sunday. But Saturday night, temperatures dropped into the teens, and melting snow froze on top of the third rail.

Yesterday, with the clean-up in its fourth day, track crews with shovels and antifreeze sprayers reopened the final segments, the New Carrollton-to-Eastern Market Line at noon, and the Red Line between Rhode Island Avenue and Silver Spring, at 3 p.m.

On the Silver Spring line, a train carrying antifreeze teams moved forward until ice broke its third rail contact and immobilized it. Then the team stepped down and sprayed the syrupy liquid onto the rails until contact was restored and the train could move again.

They worked with the power still on in the rail, sticking their sprayer nozzles into the 6-inch gap between the rail and the cover and being careful not to touch. "We all look out for each other when we're doing it, because we know it's very dangerous. It takes teamwork," laborer Charles Dean said as he sat in a train.

Once the line was clear, trains ran up and down it at high speeds to blow and scrape away any remaining ice. Sheard said that, if necessary, trains would run all night on outdoor portions of the system to assure service this morning.

Metro officials said they will try to develop equipment to deal with third rail icing, such as a large antifreeze sprayer on wheels. Removing the third rail cover might be considered too, though officials said that it does seem to work well against light snowfall and sleet.

Record storms are always a learning experience, officials said, noting that Friday's snowfall was the first time they had used much of the equipment bought after the 1979 shutdown. "They keep telling us this storm will only come around every 50 years," said David Short, assistant chief of track maintenance. "That was a short 50."

Rail systems in Boston and New York City, also hit by the storm, closed over the weekend, but restored most service for Monday's rush hours. About 92 miles of above-ground track of New York's 230-mile system shut down due to frozen switches and third-rail lines. In Boston, about 15 miles of the city's 70-mile system closed.

Boston, with heavy snowfall most years, has bought jet turbines to blow snow off the tracks. "The specialty equipment we have is quite expensive," said Paul DiNatale, spokesman for the city's system. "It might not be worthwhile for Washington to go to the extent we do."

Transit officials in Chicago said they learned their lesson in 1979, when three feet of snow hit the city and one major line was closed three weeks. As a result, special switch heaters were installed and special cars bought that chemically de-ice tracks, while doing some scraping and plowing.