About 50,000 people waited in line for trains yesterday at Union Station. And most of them weren't even going anywhere.

They were there for the Great American Rail Roadshow, an exhibition of new and old locomotives, Turboliners, Metroliners, Superliners, coal cars, flatbed cars, hopper cars and cabooses, sponsored by Amtrak and the nation's freight railroads.

There was even the private rail car used by President Franklin D. Roosevelt during World War II, which is now used by a railroad vice president for entertaining.

"It was great," Tom Senko said after crowding aboard a late-model Amtrak train for one of the 40-minute trips to Bowie and back, which he said was his first train ride ever. "We've always preferred the convenience of having the car or the speed of taking a plane," said Senko, 38, a U.S. Capitol police officer who lives in Silver Spring. "Now maybe we'll take the train some time. . . ."

The exhibition is staged in cities around the country, according to Amtrak spokesman John Jacobsen, in an effort to show people that railroads are "real and new" and to get "a few more people to ride them."

In its two days here this weekend the show at least got people to Union Station--about 20,000 on Saturday afternoon as well as the 50,000 that Amtrak officials said came yesterday.

Many clearly were impressed--with children carrying away balloons and adults hauling plastic bags with schedules and railroad literature. Whether they will ride the trains any more, however, was uncertain.

"I'm not sure the trains are going to be carrying people around much longer," said Andrew Revie, an Army major who lives in Annandale, "so I wanted to make sure my son got a ride on one. He likes trains. We read to him about them, but he's never been on one. I've never been on a train in the United States."

Will Revie, 3, and his friend Scott Fitzgerald, also 3, both went "Choo-choo" as the train surged ahead. The only noise from outside was the clatter of wheels over rails.

Besides Roosevelt's private car, which still has special hand rails that the paralyzed president used to support himself, the exhibit included a 1939 Pennsylvania Railroad streamlined locomotive designed by Raymond Loewy.

But most of the equipment was new--a $1 million engine with radar to keep tabs on all its wheels, a 143-foot long flat car that carries three trailers, and four double-decker Superliner cars used from Chicago to the Pacific Coast that rarely travel east because they are too high for old tunnels.

People waited in line an hour to see many of the exhibits.

"It's beautiful," Samuel Hall of Northwest Washington said as he walked under large windows curving into the ceiling of the Superliner Lounge. "We don't have anything like it here."

In the Superliner's dining car there was food on many of the tables--pancakes, fish and steaks, all sprayed with plastic to preserve them. Below, Isaac Harris, a cook, showed a reporter around a stainless kitchen.

Amtrak, a government-owned corporation, has operated virtually all of the country's intercity rail passenger service since 1971. Last year it carried about 20 million passengers on a network covering about 23,000 miles with about 240 trains a day serving about 475 places.

That made it the sixth largest common carrier in the United States, said Diane Elliott, an Amtrak public relations official. But it is still smaller than four airlines and the Greyhound Bus Co., and much smaller than the railroads were in their heydays before 1950 despite a federal subsidy of about $700 million a year.

"We get 50,000 people per month in Union Station," Jacobsen said, "and we crowded that much into one day. It would be nice to keep it up."