Pomp and circumstance once came naturally to Union Station, with its St. Gaudens sculpture, cavernous ceilings and a concourse said to be able to accommodate 50,000 people. But the federal government had other plans.

With the grand main area closed off and passengers routed through temporary wood tunnels during the station's remodeling, celebrity arrivals are rare these days. But yesterday afternoon the station played host to two distinguished visitors: Sudanese Prime Minister Gizzuli Daffa-Allah and President Mobutu Sese Seko of Zaire. Both came from New York, where they attended ceremonies celebrating the 40th anniversary of the United Nations, to meet with top U.S. officials.

"We don't have as many of these arrivals as we do at Andrews [Air Force Base] or Dulles International Airport ," said Linda M. Mysliwy, the State Department visits officer who coordinated both arrivals.

Yesterday's scene was not exactly a return to the days of elegance that saw the king and queen of England disembark in 1940 to a VIP room that had been redecorated for their visit. Both men arrived in the Metroliner lounge, which is carpeted with a black rubber mat (no red carpet) and decorated by posters cautioning against smoking and shoplifting.

Mobutu was greeted by a clapping, singing crowd of 70 fellow Zairians -- women in brightly colored batik dresses waving small national flags, men in business suits, and boys carrying welcoming banners with messages in French. A band of saxophone, electric guitars and drums played a rollicking song in his honor.

Stanchions, velvet ropes and Secret Service agents held back a small crowd of camera-wielding fellow passengers and passers-by.

A leopard cap on his head and his carved wooden scepter in hand, Mobutu paused to listen to the band, with Assistant Secretary of State Chester Crocker by his side. A television camera filmed the action for later broadcast in Zaire.

Kasongo Mutuale, Zaire's ambassador to Washington, said it is customary for Mobutu's people to greet him when he visits a foreign city "to show the president that we are living with the spirit of our country." He said the choice to travel by train was not unusual for Mobutu, particularly in light of the short journey from New York.

Daffa-Allah arrived several hours earlier, on the 2:49 p.m. Metroliner. He was met by two dozen diplomats representing such countries as Saudi Arabia, Egypt, Jordan and Zambia lined up in proper order of seniority by a young State Department aide carrying a clipboard.

Two State Department officials and the Sudanese ambassador to the United States boarded the train and made diplomatic small talk with Daffa-Allah and two aides while the other passengers disembarked, said Mysliwy. Then he arrived: Security men by his side, he shook hands with his fellow diplomats and exchanged greetings in Arabic.

Daffa-Allah described his train ride as "quite pleasant," and said he preferred it to airplane travel because "this gives me a chance to see more of the country, and see the people."

Then, no more than 15 minutes later, it was out the back door to a waiting limousine -- like Mobutu, avoiding the long trudge past construction debris that other travelers face.

The dilapidated main station was closed in 1981 shortly after it was converted by the federal government into an elaborate National Visitor Center, a decision that was widely criticized. It is scheduled to reopen in 1987 after a multimillion-dollar renovation.