Mariano Cortez drove his Plymouth to work last Wednesday at the Department of Agriculture, but as the snow piled up and the government announced that its workers could go home early, Cortez decided to leave the car downtown and take the subway to his apartment in Riverdale.

At about the same time, Mary L. O'Meara and Mildred M. Morgan, coworkers at the Overseas Education Association (OEA), looked out the window at snow-clogged 16th Street NW and decided that they'd better get an early start on their Metro ride on the Orange Line toward New Carrollton.

The heavy snow, the early work-release and the element of chance brought the three together for a terrible moment--in the middle seats on the left side of the eastbound train's front car. At 4:29 p.m. the car violently derailed, crushing against a concrete pillar. They became victims of the first fatal wreck in the six-year history of Metrorail.

Like so many others in this area, all three had come to Washington because they or their families were drawn here by government-related jobs. A week after the accident, they already were separated by hundreds of miles.

The body of Cortez, 46, an Agriculture Department auditor described by friends and fellow workers as a quiet, dedicated civil servant with a whimsical sense of humor, had been shipped back to New York City, where the Brooklyn native will be buried today.

The body of Morgan, 71, a semiretired secretary whose vitality made her seem much younger, rests in a Kalamazoo, Mich., funeral home. Her funeral was delayed until Friday because of the recent heavy snows in the Midwest.

O'Meara, 25, a valedictorian at Elizabeth Seton High School in Bladensburg who received her law degree last year and planned to marry this summer, was buried Saturday surrounded by hundreds of mourners at Resurrection Cemetery in Clinton.

The Air Florida crash that killed 78 persons just a half-hour before the subway derailment has overshadowed the Metro tragedy for many--but not in the minds of families, friends and coworkers of the three other people fatally injured that afternoon less than a mile from the air crash site.

"We lost two extraordinary people," said Ronald Austin, executive director of the OEA, who had known O'Meara and Morgan for three years. "A lot of tears were shed over them."

Others at OEA had taken the day off because of ominous weather reports, but the two women--O'Meara, the deputy legal counsel, and Morgan, a secretary--were among a reduced work crew that anxiously watched the snow, according to Sandra Kennedy, a coworker. The two women did not usually ride the subway together because the younger O'Meara kept longer hours than Morgan, who worked just three days a week, Kennedy said. Located in a small downtown office with only five full-timers, the association is a division of the National Education Association and represents teachers working overseas.

Both women boarded Metro at the McPherson Square station after calling ahead to tell their families they were leaving early. Morgan was to ride eight stops to Potomac Avenue and catch the bus to Hillcrest Heights shopping center, where her husband William, 72, waited in a restaurant to pick her up. O'Meara planned to ride the length of the Orange Line to New Carrollton, where her father, aerospace engineer Thomas O'Meara, would be waiting, as he often did.

Mary O'Meara was one of seven children in a tight-knit, religious family, according to her brother-in-law, Christopher Doerrer, a dentist from Hyattsville. The family moved here from New York in 1967 when Thomas O'Meara was assigned by the Grumman Corp. to work on a project for the National Aeronautics and Space Administration.

Her coworkers described her as a bright, outgoing woman who lately was preoccupied by wedding plans: She was to marry a New Jersey banking executive this summer. She would bring swatches of fabric to work so Mildred Morgan and others could help her choose those that were right for the wedding.

She had worked as a legislative aide to Maryland Del. Frank B. Pesci between 1975 and 1980 while at Loyola College of Baltimore and Catholic University's law school.

"We have lost the best in the family," Doerrer said, "She sort of tied everyone together . . . Every family seems to have one special person who is cheerful and helps work out everyone else's problems. She was that person."

The O'Mearas worship at St. Mary's Catholic Church in Landover Hills, where Mary was active and where, just two days before the crash, her parents had led a discussion group on the subject of coping with family tragedies, Doerrer said.

The night of his wife Mildred's death, William Morgan watched the account of jetliner crash on television. When it was interrupted to report the Metro crash, Morgan said later, he figured his wife of 40 years must have been on an earlier train and would arrive any minute. But the hours wore on and the victims still were not identified. Morgan said he did not learn the truth until after midnight, after he had filed a missing person's report with Prince George's County police.

William Morgan and Mildred McCay had met in Hawaii in August 1938 beneath a huge banyan tree on Waikiki Beach, he recalled. He worked in Hawaii as a machinist and engineer. She was vacationing from her job in the Kalamazoo Public Library. Morgan said he and the tall, dark-haired woman corresponded for three years before she moved to Hawaii, where they were married and lived until 1955.

Mildred McCay Morgan, a 1932 graduate of Western Michigan University, became a public stenographer at the Honolulu Hotel, where she did secretarial work for vacationing businessmen and politicians--including former President Harry S. Truman and the young John F. Kennedy, according to her family.

The Morgans moved to the Washington area in 1961, when he was offered a job on a newly formed earth satellite project of the National Oceanographic and Atmospheric Administration. They raised two children, Ronald Morgan and Louise Morgan Cope, both now 36, and worked at several secretarial jobs, Morgans said. She was fond of Washington, he said, except she complained about the hot weather.

"She was a real inspiration to everyone here," said Morgan's coworker, Sandra Kennedy, 36, who said younger women in the office were "almost embarrassed" by Morgan because she appeared more energetic and physically fit than they were. "She felt it was important to have a positive attitude about yourself and others," Kennedy said. That attitude, plus eating wisely and taking long walks, helped her look more like 50 than 70, Kennedy said.

As the two women sat together on the Metro, Mariano Cortez, a stocky middle-aged man with thinning hair, climbed aboard. Cortez rarely used Metro, according to a coworker at Agriculture, Cal Watkins.

Cortez, known as Joe to his friends and Sonny to his family, was a GS13 with Agriculture's packers and stockyards branch. Cortez also served as coordinator for the department's relations with its Spanish-speaking employes, Watkins said.

Outside work, Cortez was a bachelor with varied interests, acquaintances said. He had recently taken college courses in a number of subjects, including electronics, auto mechanics and karate, according to his sister, Mary Morin, of New York City. Robert Vance, Cortez's karate instructor at Prince George's Community College, said his student was enthusiastic and a quick learner. Cortez had finished third in a karate tournament a month ago on the Eastern Shore, he said.

Cortez was quiet and well-liked, friends and family said. When his family arrived from New York this week to clean out his apartment, his sister said, "We found all these cards and letters from people thanking him for favors, like visiting people in the hospital or helping them out . . . we found out a lot of nice things we didn't know."