Metro said yesterday that emergency officials erred in keeping about 18 people aboard an immobilized Blue Line train near the Potomac Avenue station for two hours Tuesday evening and said it is taking steps to assure such an incident is not repeated.

Metro operations chief Theodore Weigle said Metro and D.C. Fire Department officials on the scene kept thinking that a power shutoff that had stopped the train was about to be corrected and the train would continue on with little delay.

Officials wanted to avoid the risk of leading people on foot through 1,300 feet of tunnel to Potomac Avenue, Weigle said. Metro's basic safety philosophy is that in an emergency, passengers are usually safer inside a car than outside in a tunnel where there is the danger of hot rails and passing trains.

But in this case, Weigle said, people should have been evacuated earlier. Metro will review procedures to make sure passengers are kept on stopped trains only for a "reasonable" period. "We regret very much the inconvenience caused," he said.

The incident began after an unidentified elderly woman fell or jumped in front of an inbound Blue Line train at about 5:25 p.m. Tuesday in a tunnel a few yards from the Eastern Market station. The woman was killed.

Metro technicians shut off the 750-volt power on track in both directions as a safety precaution. The cutoff stopped another inbound Blue train following the stopped train. It rolled to a halt midway between Eastern Market and Potomac Avenue. Emergency lights powered by on-board batteries lit up the cars, Metro said, and Metro police and supervisors were present with the passengers.

Chief McEldon Fleming, the D.C. Fire Department's liaison officer with Metro, said officials in the cars ascertained that "there was nobody in immediate distress." The passengers "understood that they were working to correct the problem," he said. Though one woman complained of discomfort toward the end, people were "remarkably calm and understanding," Fleming said.

Weigle said the delay was due in part to safety measures implemented after a Metro train derailed last January and killed three people. In that accident, power that had shut off automatically when the train derailed came back on while people were still in the tunnel.

Under the new procedures, power is first "commanded" off from the control room in an emergency. Then technicians walk to electrical switches in the tunnels and manually turn them off for double protection. To restore power, track workers must return to each switch and reset it, which adds time to the procedure.