The largest assortment of private railroad cars ever assembled reposes on Union Station tracks this weekend, like a conflutteration of old beauties with velvet chokers on the throat and diamonds in the hair.

It comes as a surprise to the subway and quick-food set that any private cars still exist, but about 200 do, fitted up and ready to hitch on to the nearest Amtrak, and 41 of them are in town. They can be seen during normal hours today, though most cannot be entered, and from 8 a.m. to noon tomorrow, many of them will welcome visitors to walk through.

Formerly tycoons went in for private cars and so did presidents. I shook hands with President Roosevelt in one of them, stopped out in the middle of Montana 50 years ago. (It was not until 1942, however, that his car got three-inch glass and other protective refinements).

Today the cars may be owned by ordinary folk. Or, if not ordinary, at least not fabulously rich. Ross Blake, a pharmacist from Charlotte, said his car is not one of the really grand ones, just an old Pullman sleeper. He rounds up some buddies and they split the cost and it's not bad at all.

Owners have formed an association and meet occasionally for pleasure. William Claytor, president of Amtrak, invited them to meet here.

"Cost $1,500 to come to Washington from Charlotte," he said. "When you split it up with 10 or 15 guys, you do fine."

He said wives give no trouble to men who buy such cars, once they get used to the idea. His wife is with him, he said, shopping all over Georgetown, and the kids are all running around the Smithsonian. He himself seemed content to be with the train and his friends who share his passion.

Dante Stephensen of Atlanta, who owns and runs a jazz bar, and who lives in his car called The Survivor, was holding forth with some gusto yesterday, offering drinks to reporters if they could prove they were over 19 (and one could offer no proof) and soliciting quarters to go in his German slot machine of 1926.

"Actually, it was built for the English, and it only takes thruppences, which are not made anymore. The Bank of England let me buy a bag of 640. You give me the quarter and I give you a thruppence and if you win I throw you off the train."

Sounded fair enough.

He has a big sign, Beware of Dog, but there isn't any dog. Left him (name of Tundra, a German shepherd attack dog) back home. In Atlanta the two of them live in the private car on a siding. Dante (as he calls himself) pushes a button luxuriously to summon a servant with morning coffee. Not that anybody comes (he has no servants) but he likes pushing the button all the same.

"This car was built in 1926," he said, with the air of a guide saying this cathedral bay was built in 1070, "and it was mainly used by Barbara Hutton, the Woolworth dime-store heiress. Cary Grant courted her in this car.

"How about this bathroom. That marble bathtub. Not all that big, but then how big would it have to be?"

He pointed to a brass rack from a train of 1893 in a place of honor over a door. Like most of these car owners, he's fixed things up. He hasn't changed the stained-glass skylight of the dining room or its silver-plated brass wall lights or its nickel-plated table spittoon (most spittoons sat on the floor, but not this one, which is stamped The Pullman Company on the base). The curtains are figured velvet, the carpets are soft, the bar is active and the slot machine may be making a fortune for him. Some of the private cars are rentable, at figures ranging from moderate (if you just sit on the siding and have a conference) to $3,000 a day, if you're traipsing around America with food and wine provided for a good group of guests.

Amtrak has charges for hooking you on and off, plus so much a mile, depending on the length of the trip. Some of the cars cost less than $5,000 to buy (Dante's cost $3,000 in 1971, he said), many cost about $10,000 and some cost a quarter-million.

Dante said a lot of the guys, like himself, like to fix them up themselves. He prowled about junkyards till he found the right batteries for $300, though the going rate was $17,000, he said.

Blake said it hurts considerably when you have to lay out $2,000 for a brake job, but then (this in the tone of a true philosopher) it lasts quite a while.

Jill Helmbold, college student from Cincinnati, was showing guests (from Texas, Seattle and all over) through the car. John Koutsoumpas, assistant director for the Senate's food services, dropped in to visit his old buddy.

"Us restaurateurs stick together," said Dante. "Being restaurateurs, nobody else likes us, so we just have to sort of like ourselves. Like in your business."

Seemed a good time to sort of wander back to the office.