One year ago today, a crowded Orange Line train derailed beneath the Mall, killing three people and shattering Metro's vaunted faith that it was immune to serious accidents.

In the sobering reappraisal that has followed Metro's first fatal accident, the rail system has developed new equipment and procedures, moved to improve training and abandoned a long-standing philosophy that riders are safer if they cannot open doors and leave cars on their own.

The accident occurred between the Smithsonian and Federal Triangle stations at 4:30 p.m., after a supervisor took control of a New Carrollton train that had inadvertently entered a crossover track. He was unaware its front wheels had derailed. As he backed up the train, its front car was crushed against a concrete pillar and reduced to a twisted wreck.

Investigators laid most of the blame on human error, reporting that the supervisor, the train's operator and control room personnel violated safety procedures repeatedly.

Discovery of so many mistakes led investigators to conclude that training was substandard. Metro is now remapping its rail training program, having put employes through refresher courses in rules bungled during the accident.

Control room personnel are now under orders to "assume the worst" whenever normal train rhythms are broken. Rules by which they use radios and turn off the 750-volt third rail have been tightened.

Metro has installed more tunnel phones, redesigned car escape ladders, reworked third rail circuit breakers and built a special push-cart for use by rescue teams. It has commissioned studies of control room design and tunnel ventilation.

Metro now intends to conduct disaster drills more often. The transit agency may also buy new breathing equipment for firefighters to use in smoky tunnels.

Last month, the Metro board decided to modify cars so riders will be able to open doors without the aid of rescuers. Cars now contain no escape instructions, on the grounds they could be a safety hazard in themselves if misused by pranksters or panicky riders who might touch the third rail.

Metro continues to say it is one of the safest, if not the safest, systems in the U.S. But it remains vulnerable to accidents: Last September, a train derailed at National Airport after track crews improperly set a switch. No one was hurt.