LONDON, July 8 -- With two ferocious shrieks of its whistle, a scarlet steam engine pulled away from Platform 9 3/4 at King's Cross Station this morning, carrying the world's most popular living author, J.K. Rowling, on a journey patterned on the magical train trips she created in her Harry Potter books.

Rowling will ride the train, dubbed the Hogwarts Express, for the next four days on a promotional tour for the latest and largest volume in the Potter saga, "Harry Potter and the Goblet of Fire." As tens of millions of young readers around the world already know, that's the very train would-be wizards board each fall for the trip to Harry's boarding school, the Hogwarts School of Witchcraft and Wizardry.

You might well ask why the 34-year-old author needed to embark on a special train to promote her new book. This fourth volume in the series is expected to sell about 5 million copies this year, largely through word of mouth among Harry's devoted corps of youthful fans. That means it will almost certainly be the best-selling novel of 2000. Thrillers by John Grisham, who was the best-selling novelist in English until Rowling came along, generally sell 2 million to 3 million copies in their first year. But Bloomsbury Publishing PLC, the formerly smallish British publishing house that has grown hugely on Harry's coattails, is famous for its public relations skills. Harry Potter is by far its biggest earner. And Rowling said she is so grateful to Bloomsbury for buying the first Harry Potter book in 1996--after three other publishers had turned it down--that she was willing to give up her cherished privacy to indulge the PR types, at least for a few days.

But she didn't have to be happy about it. This morning, in fact, the woman who invented a global literary phenomenon looked and sounded downright glum as she arrived at King's Cross (a real station) and headed for the Hogwarts Express at Platform 9 3/4 (a fictional platform in the Harry Potter stories, but the stationmaster created one for the day).

"It's rather mad, isn't it?" Rowling said, looking out over a vast, unruly mob of reporters and cameras from around the world. When a journalist asked if she was amazed, the renowned wordsmith grimaced and said, "I'd need a stronger word than that to describe this journey."

You'd also need a pretty evocative word to describe the long trip that Rowling has taken in the past few years. It's a life journey that was considerably more complex than the familiar media picture of an unemployed single mother who dashed off a children's book in a coffee shop and rocketed to fortune and fame.

In a series of interviews in the British media last week, Joanne Kathleen Rowling--the name rhymes with "bowling"--said she can barely recognize the person in that standard profile. There was a period, just after her divorce from a Portuguese journalist in 1993, when she hung out in a coffee shop in Edinburgh, with her daughter, Jessica, in a stroller and her handwritten manuscript on the table before her. But her education--she majored in classics and French at Britain's Exeter University--meant shewould not be out of work for long.

Before she became a published author, Rowling worked as a language teacher. She taught English in Portugal until her divorce, then moved to Edinburgh, where she was first a substitute teacher and then a high school French teacher.

She finally finished the first Harry Potter book in 1996, after slaving over it for a half-dozen years. She was thrilled when Bloomsbury bought it for a reported $15,000. Accordingly, she went along when the publishers asked her to use the initials "J.K." rather than "Joanne," for fear that adolescent boys would spurn the book if they knew the author was a woman.

Although the Harry Potter stories follow a great English tradition of boarding school tales, Rowling went to a public school and said she would never send her daughter, now 7, away to school. The author said that she was lonely in her school days and that Harry Potter's studious but brave friend Hermione Granger is modeled after the young J.K. Rowling.

With the $20 million or so she is said to have earned so far from the books--including $2 million from Warner Bros. for movie rights to the first volume--Rowling has bought a large house in Edinburgh and hired a full-time secretary to help with fan mail. But she still has no car and likes to spend time at a coffee shop on the steep, winding medieval street in Edinburgh that was the inspiration for the tales' mystical shopping street, Diagon Alley.

In addition to the rapturous response from young readers, Rowling has generally received raves from adult reviewers for her witty, eventful and moving stories. But as is standard for successful people in Britain, her story has sparked a backlash.

Literary critic Anthony Holden wrote recently in the Observer newspaper that the Rowling stories are "one-dimensional children's books, Disney cartoons written in words, no more." Holden complained that Harry Potter was stealing sales from more important works, including "my own action-packed life of Shakespeare." A week after his attack, the Observer published two pages of responses, mostly from young readers, defending Rowling and accusing Holden of being jealous.

Initial reviews in Britain for Harry's new adventure were positive. The Times of London, in the first book review ever published at the top of its front page, said "Goblet of Fire" is "funny, full of delicious parodies . . . and wildly action-packed."

If that favorable first-day greeting pleased the author, it wasn't obvious from the sour look on her face as she waded past the media herd at King's Cross to board the Hogwarts Express. She may be the creator of a great fantasy world, but today she looked like a captive in the world of hype.

"I'd really love to talk to some children," Rowling told reporters, "if I ever manage to finish with you lot."