Metro began carrying passengers again late last night between the Fort Totten and Silver Spring stations after completing repairs on track damaged by the derailment of a CSX Corp. freight train. Transit officials said they expected to operate from 6 a.m. on today.

CSX officials said they had not determined what caused 21 freight cars to derail about 4:40 a.m. Friday on track running parallel to Metro's Red Line. The accident caused no injuries but ripped sections of CSX and Metro track just north of the Takoma station.

It cut off Metrorail service to and from the Takoma and Silver Spring stations for almost three days.

"The investigation into the cause is proceeding," said CSX spokesman Lloyd Lewis. "It could take days or weeks . . . . We're more interested than anybody."

CSX was expected to complete track repairs last night.

"It's just phenomenal," Metro spokewoman Beverly Silverberg said of the around-the-clock effort by several hundred Metro and CSX workers to restore service in time for today's morning rush hour. "All the material was on hand, which made it easier to put into place."

On a typical weekday morning, about 9,500 riders board at the Silver Spring station, making it the busiest in Maryland. About 3,500 passengers get on at Takoma each morning. During an entire weekday, on average, 19,300 passengers board at the two Red Line stations.

For three days, Metro provided shuttle bus service between the Fort Totten, Takoma and Silver Spring stations. On Friday, officials said, about 24,500 passengers were shuttled by 24 Metrobuses and seven of Montgomery County's Ride-On buses. On Saturday, about 7,300 people rode 10 Metro shuttle buses. No figures were available for yesterday's shuttle service.

By 1 p.m. yesterday, Metro had replaced 800 feet of its inbound track and had completed minor work on its outbound track, Silverberg said. Cost estimates for the repairs were not announced.

The work included rebuilding the roadbed, laying new rail and ties, and installing the 750-volt electrified third rail, which powers Metro's trains. Metro safety officials were testing the track late in the afternoon.

Repair work also was under way on a 1,500-foot fence that was damaged when 21 cars of the 134-car train slipped off the CSX rails and jacknifed. The derailed train cut across Metro rails on one side and nearly struck a house in a Takoma Park neigborhood on the other.

By Saturday night, CSX had hauled off the 10 cars that had crashed onto Metro property, Lewis said. By yesterday afternoon, all but three CSX cars had been wheeled away. CSX planned to remove two more by last night.

One other car, which had fallen from the tracks onto grass beside a house at the end of Chestnut Street, was left in place. Lewis said that the car, which was on its side and was filled with flour, would be unloaded today so it would be easier to lift.

The milelong freight train was carrying automobiles, auto parts, grain, paper, vegetables and appliances, along with hazardous chemicals. Two cars, containing dangerous substances, did not derail and were quickly removed from the site, officials said.

A CSX official at the scene said the only apparent damage to the cargo was a crushed front end of one automobile, which could be seen through a set of buckled freight car doors.

CSX workers quickly replaced about 1,600 feet of track by laying new rail as soon as the old was removed, Lewis said. As cranes lifted the crushed freight cars onto special wheeled platforms to be taken away by rail, tractors and plows removed and cut up the damaged track. The old track was removed by truck, making room for the new.

CSX usually runs about a dozen trains a day over that track, just west of Metro's track. No disruption occurred in CSX service because the railroad used an undamaged CSX track east of the Metro track.