FOR AN ANTIDOTE to such melancholy chronicles as the Bill Moyers documentary on women trapped in welfare, you need look no further than Ruby Sampson, 37, of Dorchester, Mass., a star graduate of her state's superb job-training program.

She is a living, breathing, can't-stop-smiling example of how a person can escape the trap. The eldest of the 13 children of a Georgia sharecropper, she left school after the seventh grade. She had three children and spent 14 years on welfare until the public assistance people gave her the choice of going to work. Now she earns $14,000 a year (twice what she got on welfare) as an operating-room technician at Boston's top-flight Brigham and Women's Hospital.

Ruby Sampson came to Washington a few days ago in the train of Massachusetts Gov. Michael Dukakis, who took a federal program called "WIN" (for Work Incentive Program) and turned it into a dramatic Massachusetts success known as "ET" (for Education and Training).

The Democratic Party is presenting ET as a wonderful new idea. Actually, it is an old one. It began in 1967 as a Great Society scheme for putting welfare people to work. Not just work, mind you -- like raking leaves or shoveling snow, which is Republican workfare -- but real, honest-to-God private-sector wage-earning.

Dukakis, who lost his job as governor in 1978 and regained it in 1982, spent his exiled years studying welfare, which he calls "the Middle East of domestic policy." He drew up a demonstration program and put it under a program chief whose background was job-training rather than welfare. They found out that day-care and medical benefits, lost when a client takes a job, were the great disincentives for job-hunting. They offered to provide both for the trainees.

The results over the past two and a half years have made ET a model. A California version, adapted for a conservative governor, is in place, and other states are following suit.

Of the 85,000 welfare dependents in Massachusetts when the program began, 23,000 are now employed, 90 per cent of them in private-sector jobs. They give the lie to Ronald Reagan's contention, expressed to House Speaker Tip O'Neill during a recent stormy White House meeting, that "people on welfare don't want to work." Thirty-five per cent of the applicants had children under six and were not required to participate.

When the ET brochure came to her apartment in the housing project, Ruby Sampson put it aside. But she didn't forget it, and one day she marched up to her girl friend's house (she had no phone of her own) and telephoned ET headquarters.

At the time, she recalled cheerfully at the governor's press conference, she was "a mess -- weighed 180 pounds, laid around the house all day, watching soap-operas, feeling depressed and taking an aspirin every hour."

She signed up for hospital technician work -- "I always wanted to be in medicine" -- even though she had never done anything remotely related .

She began training at Dimock Medical Center, where everybody made it hard for her to fail. She had to have the names of three fallback baby-sitters so she couldn't drop out for lack of help. Her counselor made her a daily schedule, "even telling me when to take a bath." She had to be there every day at 7 a.m. -- she got transportation vouchers for the three-change bus-ride.

She stayed up until 2 a.m. studying -- and she got an average of 97.3 in her tests.

Ruby was in the training program for 44 weeks, served an internship at Massachusetts General Hospital, then stepped into her present post. The job makes her proud, makes her family proud, has changed her life. "The old Ruby," she says, "is dead."

The "old Ruby" went to work in the cotton fields at age 12, when her father died. She sang with a group for a while, got stranded in Canada, went home to Georgia and did housecleaning and cooking. She had two marriages, a child by each, and after her divorces, a relationship with a man who promised to take care of her third child -- "and never did."

The transformation of Ruby was a bargain. Her training cost $3,000. Of the $85 million spent on ET since 1982, $25 million came from Washington and $60 million in matching funds from the state. But the savings, the governor said, have been considerable -- about $70 million so far. Add in food stamps and Medicaid charges, and the income tax the employed will pay, and the total gain is $107 million.

Skeptics say Dukakis's state, with its high-tech and low unemployment rate (3.9 per cent) is uniquely well off. He says the rate was 7 per cent when ET was initiated.

In the Reagan budget, WIN gets rotten notices and is recommended for elimination. "It has not proved successful or cost-effective," say the president's budget-cutters.

The triumph of Ruby Sampson is a contradiction to that misinformation.