'The Hogwarts Express, a gleaming scarlet steam engine, was already there, clouds of steam billowing from it, through which the many Hogwarts students and parents on the platform appeared like dark ghosts."

Sure enough, swirling steam from the massive red locomotive is enveloping the waiting passengers in a scene from every cinematic and literary archetype. That boy with the red hair running excitedly down the platform must be Ron Weasley, Harry Potter's best friend. Will there be witches selling cauldron cakes on board?

In fact, there is fine china and best bacon, being ferried solicitously to the star author. The interior has the rich elegance of another age, a cocoon of dark, lovingly varnished wood, an enclosed and magical world in which to speak about an enclosed and magical world. Outside, clouds of steam billow across sodden wheat fields. Inside, JK Rowling talks of trains, the painful pressures of fame and the world's best-known boy wizard, bar none.

The luxuriously beautiful replica of the train is merely an author promotional tour, albeit more imaginative than most. The train may be the most visible part of the publicity machine, but Harry Potter fever has been cunningly fanned over the past few weeks. A series of strategies, including an over elaborate veil of secrecy and a refusal to release copies of the book to the media and book shops in advance, has cranked up expectation to new highs. In truth young readers would surely seize the books in their hundreds of thousands anyway; but playground word-of-mouth is no longer deemed enough to sell that most marketable of commodities, a new Harry Potter book.

When Rowling finally did alight at Edinburgh's Waverley Station last night, like her fictional hero, she did an impressive vanishing act. The author disappeared from the sight of hundreds of eager young fans when she neatly slipped out of one of the back carriages of the Hogwarts Express and into a waiting gold Mercedes. Once again, her publicity machine had done its job, while the author remained as elusive as ever. However, there is an appropriateness to the setting. Young Harry, now on his fourth, largest and most dangerous adventure to date, came into being on a railway.

"Harry Potter was conceived on a train," says Rowling. "It was six months before my mum died and I'd been flat-hunting in Manchester with my then boyfriend. On the way back to London, Harry just popped into my head. It was the most incredible, amazing feeling. I'd been writing for years and years, I'd written two adult novels, but I had never felt that sort of excitement before. He just arrived, fully formed." That was in 1990 (though Harry did not break into book form until 1997). A decade on, and there can be few readers in the literate western world, and possibly beyond, who are not familiar with the orphan with the lightning flash on his forehead. The statistics, constantly rehearsed - Rowling's own fortune of a rumoured #15 million, the record-breaking worldwide sales - will soon require to be revised. Harry Potter and The Goblet of Fire, itself setting records as one of the longest children's books ever published, will soon send the figures spinning upwards once more.

JK Rowling, forthright and thoughtful in her cosily lamplit corner of the carriage, views the seismic and frighteningly speedy shift in her fortunes with keen intellectual clarity and ambivalent emotions. It's the old, old story: there is a price to pay when you play the media game. On the day we speak, the Daily Mail, a paper Rowling loathes, has reported on a stalker Rowling insists does not exist and again dredged up her ex-husband from whom she is entirely estranged, to add his comments to the publicity hoo-ha.

Rowling is not without the means of retaliation, however, particularly when it comes to the media. Harry's Uncle Vernon is a grotesque philistine of violent tendencies and remarkably little brain. It is not difficult to guess which newspaper Rowling gives him to read.

It is easy to find all manner of parallels between Harry's situation in the new book and that of his creator. Harry, too, has unwanted fame. In the latest instalment, as he turns 14, he begins to feel the weight of that, knows he must face up to it. He has to deal with the media and the infuriating distortions of a ghastly tabloid hackette named Rita Skeeter (a vividly drawn caricature on whom exquisitely pointed revenge is at last taken). Rowling has always spoken of a closeness between herself and her creation (in their longing, for instance, for their dead mothers) but is he now being used as a mirror of her own life?

In fact, much of this is eerie coincidence. The seven Harry Potter books were planned from the start, and Rowling is fiercely protective of that seven-book structure - to the extent of initially turning down Warner Brothers' offer of film rights as she feared their filmic intervention would hijack and skew her own planned progression. Rita Skeeter was slotted into the fourth book long before the tabloid media began hammering on Rowling's door and seeking out her "self-confessed drug-user" former partner. Rowling insists the many effects of fame, positive and negative, affect the actual writing not one whit - with the minor exception of Rita Skeeter.

"I love Rita!" she declares. "This is the one and only time when all the exterior hype broke into the writing. Normally, and this is the absolute truth, when I am writing I feel none of it. With Rita, I did realise everyone would think it was my response to tabloid intrusion, though in fact she originally turned up in the draft for the first book. It is ironic that I come to use the character four years later, given all that has happened to me. She was not invented as a response; but I will admit that I enjoyed writing her far, far more."

Rowling has wanted to be an author from the beginning. "I can never remember wanting to be anything other than a writer," she says. "I have vivid memories of childhood, and I do remember when I was very small telling my mother I wanted to be a princess. She explained to me that it wasn't really a career option, and ever since then I've wanted to write. I wrote my first 'book' at the age of six - my mum saved it."

Now she is 34, and within three impossibly short years has become a giddyingly well-known writer. She has also come to know, after the initial euphoria of simply being published, the less positive aspects of that prominence. "The first time the press came to interview me (when the American rights were sold) I had what I can only describe as a month-long panic attack. I froze completely. Never before, or since, have I been blocked for so long. It was like being pulled out from under my stone and I didn't like it one bit."

For the third book, there was no publicity tour, for the highly practical reason that she was writing what we now know to be one of the largest children's books ever - one, moreover, she had to unpick a lot of and start over, having run up against a structural plot problem. "It was nothing to do with being Greta Garbo," she says. "I had a lot of work to do. But that was when the word 'recluse' started being attached to my name, when I started being door-stepped. That is not a nice feeling. However, I don't want to over-dramatise this. I'm not Posh Spice.

"I'm always out working in cafes and bars," she says. "That's how reclusive I am. But either Edinburgh people are the coolest, nicest and most laid-back of any, or no-one recognises me. If people do know who I am, they leave me alone; and if they do approach me, which is the exception to the rule, they are never less than lovely." It is important to stress that the Harry Potter phenomenon is overwhelmingly positive. Rowling herself is honest about the money when asked if it has become a burden: "Every day, every day, of my life for the last three years I have woken up grateful that I don't have to worry about money any more."

The appetite of children for these books, which Rowling sees as carrying a message of anti-cynicism and anti-apathy, is one of the most hopeful literary developments of recent years. The elements of negativity, though, are real and will not disappear. Would she ever walk away from the pressure? She pauses for some time: "The day I felt that Harry Potter was damaging my daughter's life, I would stop. If there were 10 million children waiting for the next Harry Potter book, I wouldn't feel very happy about saying: 'You've had your lot' but there is a child I have to put first, and that's my own."

Jessica, seven, currently seems to be taking everything in her stride. "She's a feisty little thing, and she's grown into it. She was so very, very young when all this started, I don't think she has the smallest conception of how big it is. In all honesty, I'm not sure I do sometimes. The day I felt it was not giving her a happy or normal life, I would stop."

On a day like today, it is easy to see the benign side, the pure pleasure of Potter. "In the last three years," says Rowling , "there have been maybe three days when I felt the negative outweighed the positive - when I really did think, this isn't worth a candle. Stop now." She pauses, laughs. "Only three days in three years - that's pretty good going, given everything that's happened."

Harry Potter and The Goblet of Fire is published by Bloomsbury, price #14.99. A full review will appear in Saturday's books pages.