BOARD THE HOGWARTS EXPRESS, near Oxford, England, July 8 -- J. K. Rowling, the creator of Harry Potter, insists that she does not regard herself as a celebrity. But the assertion rings a little hollow when you are traveling in a style once reserved for royalty, in a personal train full of plush and brocade, crisscrossing Britain.

Of course this train -- the Hogwarts Express, named for the train that takes Ms. Rowling's blockbuster creation to the Hogwarts School of Witchcraft and Wizardry in all four Harry Potter books -- is the centerpiece of a publicity stunt timed to celebrate and feed the frenzy stirred by the latest in the series, "Harry Potter and the Goblet of Fire," published to great hullabaloo today. And the apparent luxury -- dining car resplendent with white linen and crystal, sleeping car for Ms. Rowling and the entourage from Bloomsbury, her British publisher -- is not quite the magical ride of the novels.

The train rocks and rattles and wheezes. Its 57-year-old steam engine develops a fault and has to be towed behind a diesel locomotive. The antique cars make so much din that a reporter's tape recorder is overwhelmed with white noise during a tightly scheduled 30-minute interview in an observation car. The train's itinerary is to trundle for four days from book signing to book signing at railway stations large and small where the Harry Potter aficionados await a glimpse of the person who gave them their hero.

And at the center of all this stew of hype, stress, adulation and ever-changing deadlines stands Joanne Kathleen Rowling, a slight, 34-year-old writer from Britain's university-educated middle-class, a onetime single mother on welfare now credited with being No. 3 among Britain's top-earning women, with a reported $22 million-plus already gathered from a lightning career.

But the moment is not all triumph, and in a way this rolling monument to success says as much about modern Britain as it does about the phenomenon of Harry Potter. There is an expectation, for instance, that her success automatically entitles the world beyond the Hogwarts Express to bestow the familiar trappings of celebrity -- photographers' popping flashes, glamour to feed dreams -- as if acclaim for her writing made Ms. Rowling the same kind of public property as others might only yearn to be.

And there has been a possibly curmudgeonly reluctance in the broader literary world to allow Harry Potter -- and Ms. Rowling -- to pass by without pointing out that however Harry Potter may be drawn as a fictional persona (one respected literary editor called him a "cipher"), Huck Finn he ain't. Even as the cash registers have been ringing across the Atlantic, Ms. Rowling's work has lost out on two prestigious prizes: the Whitbread, for book of the year, and the Carnegie, the top British prize for children's writers. ("She was thrilled to bits just to be short-listed," said a Bloomsbury publicist, Rosamund de la Hey.)

Ms. Rowling's books, said the author and Whitbread jurist Anthony Holden in The Observer a few weeks ago, are "Disney cartoons written in words, no more." (The United States reaction seems more "celebratory," Ms. Rowling observed in the interview. "It's a horrible cliché, but Americans do regard success differently.")

Of course the publication of the fourth book has been mercilessly hyped. And with Warner Brothers planning to begin filming the first Harry Potter movie in the fall, directed by Chris Columbus of "Home Alone" and "Mrs. Doubtfire" renown, the exploitation of the dream world Ms. Rowling spins around the boy wizard is only beginning. But will that lead to an anti-commercial backlash? It is an issue, Ms. Rowling implies, on which she is ready to take a stand.

"I would do anything to prevent Harry from turning up in fast-food boxes everywhere," she said. "I would do my utmost. That would be my worst nightmare."

From approving the script for the forthcoming movie to the spinoffs it produces, Ms. Rowling seems to be ready to defend her vision of Harry Potter to the last. In conversations with the director Steven Spielberg about a possible Spielberg movie of Harry Potter, she said, as the train chuffed and hooted its way past the hedgerows and meadows of central England, the project never went anywhere because "this film would be my vision, and I think he felt he would he hampered in giving his imagination free rein."

And on the commercialization of the fourth book, she said, "I'm quite clear in my mind what I would like to be out there and what I wouldn't."

Ms. Rowling has sought to maintain similar control over public access to her personal life, but that has not always been possible and, much as she sought in the earlier years of her success to pretend to herself that acclaim would not change her life, it has.

Earlier this year, for instance, Britain's tabloids tracked down her ex-husband, a Portuguese journalist named Jorge Arantes with whom she had a brief marriage in the early 1990's. Ms. Rowling has brought up their daughter, Jessica, single-handed. But suggestions that her ex-husband may have helped in the creation of Harry Potter rankle with her. "He had about as much input into Harry Potter as I had into 'A Tale of Two Cities,' " she said tartly.

After the breakup of the marriage in Portugal, she returned with Jessica to Edinburgh, weighted by depression and poverty. "If you have been through three or four years of worrying on a daily basis about the money running out," she said, "you are never going to forget what that's like."

She acknowledged that she shook her depression in 1994 only with nine months of counseling, realizing later that her continued ability to write during this period was "a sign that I wasn't very badly depressed."

Finding a publisher for the first Harry Potter book was not easy either, she said, and she is still at a loss to explain what, precisely, has propelled sales of more than 30 million, most of them in the United States, a landscape remote from the boarding-school culture of Hogwarts.

"I can't explain it," she said. "I don't have an answer."

But, offering an oblique riposte to those who have criticized her use of language or the depth of her characterization, she said: "I just write what I wanted to write. I write what amuses me. It's totally for myself. I never in my wildest dreams expected this popularity."

"There's no formula," she added later.

With the arrival of the fourth book this weekend, of course, popularity has turned into feeding frenzy. Hundreds of children and their parents have waited at the railside stops, forming lines for book signings. Batteries of television cameras at King's Cross station in London -- where the Hogwarts Express departed for its four-day perambulation ending in Perth, Scotland, on Tuesday -- were so intrusive that her fans had a hard time glimpsing her. In a nation that celebrated Diana as the People's Princess and is obsessive about celebrity from soccer players to soap stars, did she feel she had joined those illustrious ranks?

No, she said. She has sometimes been recognized and has been photographed writing in her favorite cafes in Edinburgh. ("The first draft is always in longhand," she said.) But "I can go completely unnoticed down any street in Edinburgh," she said. "Celebrity is not a word I would even apply to myself at all."

Of course her life has changed: just giving interviews on a personal train underscores the transformation from obscurity. Television news has charted the sales, in Britain, of the entire record first print run from warehouses to bookshops: 1,027,000, said Bloomsbury's chief executive, Nigel Newtown.

A promotional tour in the United States is to follow in the fall. "But then I go home, and life will resume its normal pattern," she said. "It's not particularly interesting -- seeing friends, working, raising a daughter -- the most important thing in my life, Harry included."

Her newest book, "Harry Potter and the Goblet of Fire," is arguably her most ambitious. It is the longest -- 734 pages in the American edition from Scholastic -- and that is longer than even she imagined. She was late delivering the manuscript. She worked 10-hour days to produce it. She had, she said, to start over from midway through when she realized that part of the plot had not been set up to reach the conclusion she wanted. Not only that, the fourth book was designed as the culminating point to which the first three had been leading. (There are supposed to be seven, meaning three more are due.)

For the first time she touches on themes like political involvement, jealousy, fame, romance and the death of a Potter ally: all rites of passage.

"It's the end of an era in the context of the whole series of books," she said. "For Harry his innocence is gone."

She intimated that as the series progresses the mood may darken. The death of one character in the fourth book, she said, is "the beginning of the deaths."

Oddly enough, though, death was not the most difficult theme to handle. "I don't want to disturb children," she said, "but I don't want to write about death as if it's something that doesn't happen." And after all the whole series begins with the death of Harry's parents.

So what was the hardest part?

The answer was a character called Rita Skeeter, a hard-bitten journalist with a liking for fabrication and scoops, usually blending the two into one. "I knew people would assume that this was my response to what's happened to me," she said. But she decided to go ahead with the character anyhow.

One question that begs asking after Ms. Rowling's success in the United States is why none of the characters are Americans. In the latest books the reader is introduced to European wizards, and there is even a passing reference to African and American wizards. But Ms. Rowling, who calls herself "a very British person," insists that "you are not going to get an American exchange student brought in at Hogwarts."

"I don't think it would be faithful to the tone of the books to have somebody brought in from Texas or wherever it might be," she said.

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