Metro engineers trying to figure out why a 72,000-pound subway car jumped its tracks Monday night on a rail bridge 30 feet above ground remained puzzled one day after the rare derailment.

Transit workers spent much of yesterday struggling in the snow to restore full service to the Blue and Yellow lines, which had been severely diminished by the derailment just north of the Reagan National Airport station. Trains had to be single-tracked around the accident site, creating serious delays for thousands of passengers trying to move between Virginia and the District.

Once normal service was restored at 4 p.m., Metro officials focused on why the last car of a six-car train left the tracks as it departed the airport station and rolled onto a rail bridge headed toward the District.

"We don't know exactly what the answer is," Lem Proctor, Metro's chief operating officer for rail, said after spending hours on the snowy track bed.

The National Transportation Safety Board is monitoring Metro's investigation and receiving updates from the transit agency but is not conducting a probe at this time, spokeswoman Lauren Peduzzi said.

Proctor would not comment on whether cold temperatures might have played a role in the accident by causing the steel rail to contract. The temperature at the airport was recorded at 29 degrees at the time of the accident. The last time a Metro train carrying passengers derailed was in 1998, when heat caused a rail to expand and buckle and one car of an Orange Line train left its tracks. That time, as in Monday's incident, no one was injured.

Workers planned to slice a 20-foot section of steel rail where the derailment occurred and take it last night to the Alexandria rail yard for detailed inspection. Engineers examined the rail on its track bed yesterday but could not spot any defects, Proctor said.

The rail car involved in the incident, manufactured in 1985 by Breda Construzioni Ferroviare, was moved early yesterday to the Alexandria yard, where the wheels that derailed are being dismantled and closely inspected, Proctor said. They will measure the diameter of the wheels, take the gauge measurement and take apart the suspension system components.

Track walkers inspect every piece of Metro track twice a week to look for signs of a defect. Proctor said the stretch where the wheels jumped the track was last inspected the morning before the derailment. A more comprehensive track inspection, performed with equipment that uses ultrasound waves to detect rail weakness, is done every three months. Metro officials yesterday could not provide the date of the last ultrasound inspection for the track under investigation.

The derailment occurred after the train left an interlocking north of the station, about 150 feet from the platform. Derailments are more likely to take place around interlockings, also known as switches, because that is where gaps in rail occur as opposed to the uninterrupted steel of the railroad.

The last pair of wheels of the sixth car left the running rail and veered to the left, knocking into the electrified third rail and an iron safety railing. About 60 passengers were aboard Train No. 410 when it derailed about 8:20 p.m.

The operator, Phillip R. Murphy, learned about the derailment from a passenger in the last car, who hit an emergency intercom button to alert him after the car started jolting from side to side. There is no alarm in the operator's console that signals if the wheels have left the track.

Murphy called Metro's Operations Control Center on his radio to repeat what the passenger had told him, and train controllers in the center ordered him to stop the train, Metro spokesman Ray Feldmann said.

"We went quite a distance and the window broke because we had smashed into the railing," said Parker Taylor, 24, a law student riding in the last car. "I tried to find an emergency brake, but there was none. Oh my God, we just couldn't stop. Have you ever been to Universal Studios in L.A.? It was like that earthquake ride. And we were on the elevated part, so we could have gone over."

The derailed car was dragged about 1,500 feet by the train, shearing off the safety railing and smashing ceramic insulators while also shredding the third rail's fiberglass cover, before the operator applied the brakes. Metro officials estimate that the train was moving at or slightly below the speed limit in that interlocking, which is 22 mph.

"The operator, who was six cars away from what was going on, didn't immediately know what was happening," Feldmann said, adding that Murphy appeared to follow proper procedure but that his actions are being scrutinized as part of the investigation. Murphy has worked for Metro for six years and has been running trains for nearly three years, Feldmann said.

The passengers were evacuated through a door in the third car and were led by Arlington County firefighters and Metro Transit Police over a makeshift walkway built between the car and the ground.

"By the time the police and fire people showed up, they were really good at giving us information," said Soren Anderson, 24, a nonprofit researcher from Arlington. "I never really felt in danger. It's a little unnerving to know it started while we were on the top of that bridge."

The rail on the track bed and the third rail appeared unscathed, but workers had to replace the safety railing and third-rail insulators. The rail car suffered little damage.

Monday's incident took place just south of the site of a derailment last Wednesday in which a piece of maintenance equipment came off the tracks between Crystal City and Pentagon City. The equipment is a rubber-tire vehicle that can ride on rails. The vehicle blew a tire and derailed. Proctor said the flat tire was not caused by the track and the two derailments within a week of each other are not connected.

Staff writers Sewell Chan and Katherine Shaver contributed to this report.

Commuters get a free ride to the Pentagon stop after the derailment of a Metro rail car on the Blue Line just beyond Reagan National Airport.Workers survey the tracks where the train derailed near the Reagan National Airport station. They planned to take a 20-foot slice for examination.Commuters from the Franconia Metro stop crowd a bus heading to the Pentagon. Metro crews spent much of yesterday restoring service.S.G. Richards, Metro supervisor, helps commuters get on buses at the Franconia stop.