Honestly, the damage mothers do. Why, Bruce Greenberg might be a lawyer or a professor today if his Mama hadn't insisted. But down went her foot. She said Bruce absolutely, positively could not have that expensive model train for his sixth birthday.

Twenty-six years later, both youth and Freud have been served. Bruce Greenberg is one of the highest model train fans in America. What's more, it's his living.

And Mama? Bless her, she has become a stockholder.

The object of her affections is the Greenberg Publishing Co., based here in the foothills of the Alleghenies. Bruce Greenburg serves as its president, counsel, buyer, bottlewasher and whatever other title might come up tomorrow.

The company offers more than 1,00 book titles for sale, as well as knickknacks, coffee mugs, patches and caps. Almost all deal with model railroading in some way.

Want a color photo of Lionel's best 1935 engines, all done up in peacock blue? Greenburg's is the place. Want a manual that will tell you how to build or break down every American Flyer ever made? Greenburg carries it. For the wistful, there is even a longplaying record of nothing but railroad whistles.

What there isn't any more is Greenburg's massive collection of trains. By late 1975 it totalled about 800 individual cars, not to mention innumerable yards of track, switches, coal chutes and plastic pine trees. It was, says its amasser, "one of the best in the Washington area, a true thing of joy."

But late '75 was also when Greenberg left his career as a university administrator. So he sold all but a couple of dozen of the goodies in his collection. Those two dozen remain on active duty in a warehouse here. The others fetched a grubstake business, ahern, on the track.

But let us not group Greenberg with ladies' ready-to-wear salesmen, fertilizer wholesalers and other businessmen to whom the money is the message. Model train publishing pays nicely - don't get Greenburg wrong. But bucks are far from the larges reward, or the chief purpose.As Greenberg says, this is "not a job, it's a passion. It means something."

The Greenberg eyes bear witness to this.In calm times, they are searching, and somewhat to the light side of peacock blue. But talk railroading with Greenberg, and the pupils widen, the blue turns stormy and oceanic. When Bruce Greenberg says the memoray of his sixth birthday disappointment "is very intense," his glare says it louder than his larynx.

What hath such passion wrought? A business that grossed "at least $100,000" in 1977. A reputation as a train guru, ever ready to swap tales and advice by long distance. And a van that takes Greenberg, his wife and his two children to a train show almost every weekend of the year.

It was not always thus. In fact, Greenberg admits that his boyhood infatuation with trains gave way during his teen-age years to stamps and tropical fish. "Call it a series of passion," he says.

But one day in 1967, while Greenberg and his wife Linda were graduate students, they noticed a classified ad in a newspaper. A Michigan machinist was offering some trains for sale. "Little did I know," says Linda Greenberg.

Plunk went $30 into the machinist's palm for a rather ordinary engine, four cars and a circle of track. A few more plunks, and it was goodbye to the $100 a month the Greenbergs then had in uncommitted income.

Both Greenbergs say it never became a choice in those days between a buying a meal and buying a caboose. But Bruce Greenberg does say he and his wife have held "ongoing negotiations an discussions" since that time.

Clearly, Bruce Greenburg was a master negotiator and discusser. In a very short time, the Greenbergs owned several dozen pieces. But many were dull or duplicates."Obviously," says Greenberg, "the roktian ws to sell the ones I didn't want and buy others."

Obviously?

"Well, it was obvious to me."

So Greenberg plunged on , consistently building his collection as academic jobs took the family to Galesburg, Ill., then Columbia, Md. It was in Columbia, in 1974, that Greenberg first entered train-book publishing as a hobby by duplicating an old Lionel catalog. The next year, it was full-time.

Selling his prized train collection was not traumatic, Greenberg insists. "The fun had been in finding them, acquiring them," he said. Never again will he have such an extensive collection, he vows, "because it's a nuisance, it takes up too much room."

"Yet Greenberg still talks in terms of "toy train empires." The appeal will never be lost - to him or succeeding generations - "because it's an area where one has complete control, in a society where one has relatively little control."

Greenberg's goal, beside endless expansion "of his business, is to "make trains the third major collectible in the country," behind stamps and coins. "The toy train will prosper as a toy because of the inherent fun in it. Seeing the world in miniature is very satisfying."

And miniaturizing the world for profits is, too , says Bruce Greenberg. "Your own independence, your own business. This," he says, "is the American dream."

Besides, he gets to run his son's trains in the playroom.