NEW YORK, OCT. 12 -- Real estate developer Donald Trump today bought Eastern Air Lines' shuttle for $365 million in cash, promising to make the new Trump Shuttle between Washington, New York and Boston "a diamond, an absolute diamond."

Frank Lorenzo, chairman of Texas Air Corp., which owns Eastern and Continental Airlines, said he has no current plans to sell the remainder of Eastern. But he was careful to leave open the possibility of renewed talks with Trans World Airlines owner Carl Icahn, who has expressed an interest in buying some or all of Eastern's planes and routes.

The shuttle sale, which includes 17 airplanes and access and ground rights at the three airports served by the commuter service, is to take place Dec. 15, although several of Eastern's unions are expected to challenge it in court. The sale is also subject to approval from the Department of Transportation and the Federal Aviation Administration.

Lorenzo and other Texas Air officials said they foresaw no serious legal or regulatory obstacles to the sale, including the union challenge. Financing the sale is also not likely to be a problem: Just this week, Trump was named one of the 400 richest people in America by Forbes magazine with a personal fortune estimated at $1 billion.

As part of the deal, Texas Air signed a marketing agreement that will allow anyone riding the Trump Shuttle to participate in Eastern's OnePass frequent flier program. In addition to Eastern, the program allows passengers to redeem mileage on Continental, SAS, Air France and Lufthansa.

Trump, pointing to a red-white-and-blue model of a 727 jetliner with the name Trump emblazoned across the tail in large red letters, repeatedly emphasized his plan to provide quality service for shuttle commuters.

"I'm looking to do a really wonderful job with the shuttle," said the 42-year-old billionaire, whose expensive tastes and lavish life style have become a trademark. "When you're flying this shuttle you're going to be flying with something that's quality and good and on time."

Trump also added fuel to recent speculation that he may expand the shuttle service to Atlantic City, where he owns two major hotel casinos and is trying to buy a third.

"It's something we're looking at," he told reporters in an ornate marble-and-gold-leaf anteroom at New York's Plaza Hotel, which he bought earlier this year. "Flying people in {to Atlantic City} would be an interesting idea." Trump said the shuttle would use the nearby Pomona Airport if he decides to expand service to the New Jersey resort.

But even without an Atlantic City connection, Trump said the shuttle will be a good investment. Analysts estimate that the three routes make between $30 million and $40 million a year. At current fares, the shuttle's yield is more than 50 cents a mile, compared with an industry average of 11 to 12 cents a mile, according to Robert J. Joedicke, a senior analyst with Shearson Lehman Bros. Inc.

Eastern's unions were quick to challenge Lorenzo's latest move, as they have ever since he took over Eastern in 1986. The unions argue that, under federal labor law, the company must negotiate any major changes at the airline. Texas Air officials disagree.

As part of the sale agreement the 850 shuttle employees will be offered the option of joining the new company with the benefits of their existing union contracts or staying with Eastern.

Joseph Guerrieri Jr., an attorney for the International Association of Machinists, said he will seek an injunction against the transaction this week in U.S. District Court. He described the deal as "truly the death knell for Eastern" because it eliminates its only consistently profitable operation.

Texas Air paid $660 million for all of Eastern two years ago. According to figures released today, Lorenzo and Texas Air will have a $240 million profit on the sale of the shuttle alone -- money Lorenzo said will provide "a major capital infusion" for the remainder of Eastern's operations.

Since the fall of 1986, the Eastern shuttle has faced competition from the Pan American shuttle, which now accounts for about 45 percent of the lucrative shuttle market. Pan Am positioned itself as a better-quality product against a shuttle that was then legendary for its cattle-car atmosphere.

Pan Am said yesterday that it welcomes Trump's entry into the northeast shuttle market. "Our experience over the last few years has shown that more competition results in an increased service level for the customer," said Pamela Hanlon, a spokeswoman for Pan Am.

"The most intriguing question is: Will Trump provide a higher-quality product?" said Julius Maldutis, an airline industry analyst with Salomon Brothers.

In addition to acquiring the shuttle, Trump also acquired the services of Bruce R. Nobles, who will become president of the Trump Shuttle. Nobles inaugurated Pan Am's shuttle as its president before moving over to Texas Air earlier this year.

Notwithstanding Lorenzo's assertion that he wants to continue to operate Eastern, industry analysts said that the sale of the shuttle appears to set the stage for the sale of additional assets by Eastern.

"It cuts the heart out of the airline in a sense," said Joedicke. "That's been the consistent money-maker in Eastern." A sale of the remainder of the airline wouldn't surprise him, he said.

It wouldn't surprise the unions either. Throughout the litigation against Texas Air and Eastern, the unions have contended that Lorenzo is dismantling Eastern, by transferring its valuable assets to its nonunion subsidiary, Continental Airlines, or by selling them off.

"The announcement today is just another episode in the Texas Air chainsaw massacre," said James Linsey, an attorney for the Air Line Pilots Association.