LINKS. Pass it on

Once upon a time when a young wife in the Midwest with a healthy, noise-oriented boy of 20 months, plus a brand-new baby, migrated to Texas to join her husband in their about-to-be home, she had to travel first to Chicago to catch the train on which she had reserved an upper and lower berth. Her trip would take three days. With, of course, the two nights in between.

It wouldn't be too difficult, she figured, because her son would sleep in the upper while the baby, doubtless, would nap all the time, content with the powdered milk someone had suggested she use on such a long trip.

But the baby did not care for powdered milk. And one aisle could become impossibly confining for a small boy with gobs of energy.

The other passengers, mostly businessmen, were pleasant. They did not complain (out loud) of the children's noise. The porter tried to be accommodating.

But the older child simply refused the upper berth. He clamored to be with Mama, right between her and that baby. Never before had he behaved like this.

Of all the other travelers, the one who most perturbed her was a woman in her early 40s, perhaps, who was perfection itself. ''That suit she's wearing - it costs more than all my clothes in the last 10 years. And her hands - beautiful, with nails that look freshly polished every half-hour.''

Without doing anything except read her book, without saying a word except ''thank you'' to the porter who catered to her although she requested nothing, this woman was intimidating.

So on that last morning, when the young mother wasn't certain she could hold out during the final hours of the trip, when she was afraid that biting her lips would no longer keep her from screaming along with her children, it was then she saw the immaculate lady coming down the aisle, aiming for her and the children.

The young mother tried to shrink into invisibility.

To her astonishment, the lady reached out and lifted the baby across her own shoulder. She clasped the little boy's grubby hand, along with a couple of diapers from the handy pile in the corner of the seat. She told the porter to bring pillows and covers.

Also, he was to make the young mother comfortable so she might rest for the remainder of the trip.

She quieted both children and she kept them quiet. How she did it, or what she did, was impossible to note, since the young mother was sound asleep. Rested ... loving her youngsters again, ''I can't tell you how grateful ...''

''Don't. Now you can meet your man with a smile.''

''You've helped - so much.''

''No more than I was once helped when I needed it. Then, I tried to say 'thank you' but was told that someday I'd see somebody who needed my help. I was to pass-it-on. Now I say to you, 'Pass it on.' ''

Naturally, the young mother vowed to do just that, but in the minutiae of daily living that difficult train trip dissolved, faded away.

Some years later there was a war during which it became difficult to buy necessities such as gas for the car. Ration tickets - what a luxury! One walked. Or stayed home. But when it was a matter of visiting an aged parent it was necessary to take a chance on the condition of the tire.

So now, there they were, the family standing by the roadside, halfway there, the right rear tire as flat as an ink blot.

In the distance only one building was in sight. A tiny house behind huge lilac bushes set way back on the country road. Did it have a telephone? Whom to call for help? Was there a town nearby? They plodded down the road to the small house.

Imagine their relief when one of the nicest smiles ever answered their knock on the front door. It belonged to a short, plump woman who listened intently to their tale of woe. She called to ''Charles'' - a telephone pole of a man with a voice like a mouse's squeak who, he assured them, had friends in the town seven miles away who'd help. Naturally, no place of business was open today. But they were not to worry. He'd drive them there and back. He'd manage everything for them.

Later, after milk, homemade doughnuts, and an adequate tire ... oh yes, they could pay for the tire but not for anything else.

No, they should just pass-it-on ...

Three words that stunned.

But the Little Missus, as he called her, explained, ''Someone will need help more from you than we need the thanks. I've always thought the nicest jewelry a person could have are the golden chains which link one to another as you pass-it-on. You never know the value, only that it gives joy to be a connection in this kind of a bond between people.''

There were miles and miles, there were years and years between the lady on the train and the housewife in the country but they were equally ardent in their formula for satisfaction.

You know something? It makes sense.

Now the young mother (no longer so young) has been able to employ the phrase several times - and it's fun. But a word: The passingm flourishes most fruitfully when the passer and the passee are strangers. With anonymity there is no sense of obligation. The freedom of the act. That's important.