It happens every day on the Bust-Out Special, the subway train from Times Square out to Aqueduct: sweet, silver-haired Mother Machree seats herself primly, unfolds the Racing Form, whips out a ball-point pen and becomes Hard-Hearted Hannah.

So I should have expected it. Four hours earlier the resident 12-year-old had entered trippingly into Bowie Race Course, pink-cheeked, innocent and eager for his first day of a bright new sport. Now he was trudging off toward the Gray Line bus, surly and red-faced, kicking paper cups and uttering words I hadn't thought he knew.

He had become, in 10 easy races, a horse-player.

"Racing," wrote Joe H. Palmer, the most splendid writer who ever elected to work for newspaper pay, "he never claimed to build character; it develops what a man has, be it good or ill." What, then, had I nurtured?

It was time the boy should know this, the most natural of our sports, with roots in times when man cherished his horse as an extension of himself - his sustenance, but his pride, too. The horse remains a symbol, Joe Palmer taught, of a way of life that has among its elements grace and charm. It is a difficult ethos to isolate at pragmatic Bowie, but an attempt was made.

We went up close to see the horse saddled for the second race, and learned: that a horse blanket is called a "piece," that there is virtually no such thing as a black horse, that a mile race cannot be run on a mile track. All that esoteric stuff. Now we were watching the race at the finish line, on what is called the "lawn," whether it is concrete or asphalt. After the race, the boy said, he would like to watch the jockeys weigh in.

Then Lorraine The Queen charged by, winner by three-quarters of a length. "Let's go," the boy said. Where? "Cash my ticket," he said. He was reminded that he wanted to watch the jockeys weigh in, and that no money would be paid until they did.

"This is the life," the boy said, pocketing his $9.80. "Shall we," he said, a 90-cent hot dog later, "go and get some upstairs information?" Upstairs, handicapper Clem Florio supported the paternal judgment that the thied race was a good one not to bet. "But he said if you have to play the race, Amusement Park is the best bet, and that's my horse."

The puristic game plan was to shield the boy from "exotic" (read it shlock) bets like the exacta. But the third race, and the fourth, fifth, sixth and seventh, were the kind of events winter racing brings: six horses, with the longest shot 10-1. They make exactas (picking the 1-2 finish) seem reasonable, or a reasonable altenative. In the fourth, it was suggested, Vavacity at 15-1 seemed an overlay, a nice kind of exacta horse. "He's got no shot," the boy declared in purest race-track patois. He had been a race-track for 61 minutes abd already he was talking in hyperbolic absolutes.

It was ll over in 240 minutes, but the boy lived a lot. The ordeals of a horse player are infinite, but he experienced a number of them.

His first-race choice was the best horse, but the intimidated apprentice jockey's ill-judged ride was a Great Circle route to second place.

Then the same jockey gave the boy a good ride and his maiden victory in the second.

In that race our young handicapper eliminated the obvious favorite, Weezeelee, on the grounds that King Leatherbury, the world's winningest trainer in 1977, "may be the best but he can't make a horse like slop." The track in fact was muddy-fast; the "slop" presumption on the bus to Bowie was attributable in equal parts to the weatherman, the Fairfax County school system (closed) and the boy's father.

Another first-time experience: He was shut out, from betting on Amusement Park in the third, because his father was explaining how many yards there are in a furlong when the gate opened. This made no impression because Amusement Park was a nolo-contendere fourth, from start to finish.

By the seventh race the boy had decided that (a) he hated exacts but (b) he was going to cash a ticket no matter how little it paid. His 3-5 exacta might have been the shortest price of the day, but the order of finish was 3-6-5. Then still another vista opened: The "INQUIRY" sign lighted up on the tote board. If No. 5 was claiming foul against No. 6, it was explained, and if the claim were upheld, the 3-5 exacta would prevail, perhaps for a $20 payoff. But if the claim was against the winner . . . It was, and the 6-5 paid $29.40. "A gyp," the boy concluded.

To make his maiden venture complete, the boy had a Good Thing in the 10th. Early in the day, on the grandstand lawn, the father had tried to explain abut tips: how a whisper at the manure pile at daybreak turns a 20-1 shot into an odds-on favorite by post time.

But if this horse didn't win, the trainer had said, then he didn't know anything about horses and he'd quit. The steed, 10-1 in the program line and flattered at that, went off at $2.40 to the dollar. And he ran, taking the lead after a half-mile, keeping it for 200 yards or so, but finishing fourth.

And there, the boy growled on the way to the bus, went all but four bucks of the $20 bill that was a 12th-birthday present. "You can write in the paper," he snarled, that Devin Mann is teed off."

Clearly, it was time for the good news: that I had planned to give him $20 to play with, as a laboratory, but that it seemed a good idea to let him use his own because nobody, at 12 or any age, ever learns anything from betting money it doesn't hurt to lose - because that is not gambling. He would, I assured him, be made whole. I did not mention the ten bucks or so I had expended in buying the reciprocals of his exactas. Suicide insurance, they call it, but we could go into that another day.