There's little Metro can do to prevent the snow problems that felled 120 rail cars this week, short of an expensive, complete remodeling of every car that would move air ducts from the undercarriage to the roof, a top Metro official said yesterday.

"Is anybody going to invest that much money into cars for a couple days of snow that happens every four, five years?" said Lemuel M. Proctor, Metro's general superintendent of rail car maintenance. "If you're a rider, you think, 'Sure, why not? Why not build a chimney on the cars?' But if you're a member of the board of directors, you might not think that."

As the weekend approached, Metro got a handle on its problems. It restored 48 of the stricken cars to service yesterday and expected most of the remainder to be back on the rails today.

Commuters still found crowded platforms and rail cars yesterday, but conditions were nothing like the chaos that characterized Thursday's commute, when the subway system collapsed with ridership higher than normal and too few rail cars. About 20 percent of the cars in Metro's rush-hour fleet had to be taken out of service after snow and ice accumulated in cooling systems, Proctor said. That forced Metro to run four-car trains during rush hour instead of the usual six- or eight-car trains.

Passengers were left with three options--fighting their way into overflowing trains, waiting on packed platforms for up to 90 minutes for a train with space to board or abandoning the effort in favor of a cab or automobile.

The problem struck every type of subway car Metro owns, regardless of manufacturer or age. Blowing snow, sucked into the cars through the cooling system mounted on the undercarriage, landed on sensitive electronics inside the motor system. When the snow accumulated and melted, the water caused electrical shorts and blown fuses.

Metro spent more than $1 million after the January 1996 blizzard on snow-fighting equipment, including filters that prevent snow and ice from reaching traction motors under rail cars. During Tuesday's storm, those filters worked well in keeping the traction motors dry, but they couldn't protect the cooling systems, Metro Deputy General Manager Charles Thomas said.

Subway systems in every other cold-weather city experience the same problem, Thomas said. But because Metro has no buffer--it owns just enough cars to handle rush-hour ridership--a breakdown of more than a few cars causes serious service problems. The agency has purchased 110 new rail cars, but they will not be ready to roll until December.

Metro officials are bracing for tomorrow, when another storm is expected to reach the Washington area. If the storm delivers wet snow or ice, the rail cars are not likely to be affected, Proctor said. But if it brings dry, powdery snow "that blows around and piles up," more trouble with propulsion systems is expected, he said.

As for highway travel, traffic watchers said yesterday's commute was fairly typical, in contrast to the backups Thursday morning. "The normal delays are back but nothing out of the ordinary," said Ed Bowers, of SmarTraveler, the traffic information service.

In another Metro development, the transit agency said it will be late September before it replaces 20,000 electronic relays in the subway system that have been failing at an alarming rate.

Metro had expected to replace the relays--which transmit information among trains, signals and switches and are key to preventing collisions--by May. But after testing the first batch of replacements supplied by Alstom Signaling Inc., Metro found that the contacts on the relays were not working properly. Alstom says the setback will cause a four-month delay.

Replacing the relays would allow Metro to resume operating trains by computer. That would mean a return of smoother, faster rides and less wear on brakes, transit officials said.

The subway cars, designed to be run by computer, have been controlled by drivers since April, when the electronic failures reached a crisis point.

The relays, installed when Metro was built in the 1970s, were expected to last 70 years. But four relays have failed in the past two years, sending erroneous instructions to some trains. Statistically, the system should have no more than one relay malfunction every 50 years, transit officials said.

After a seven-month investigation, Metro could not determine why the relays failed and whether it was the fault of the transit agency or Alstom.