How Harry Potter fell into JK Rowling's head

J. K. Rowling in her brief biography on how Harry Potter fell into her head:

After leaving university I worked in London; my longest job was with Amnesty International, the organisation that campaigns against human rights abuses all over the world. But in 1990, my then boyfriend and I decided to move up to Manchester together. It was after a weekend’s flat-hunting, when I was travelling back to London on my own on a crowded train, that the idea for Harry Potter simply fell into my head.

I had been writing almost continuously since the age of six but I had never been so excited about an idea before. To my immense frustration, I didn’t have a functioning pen with me, and I was too shy to ask anybody if I could borrow one. I think, now, that this was probably a good thing, because I simply sat and thought, for four (delayed train) hours, and all the details bubbled up in my brain, and this scrawny, black-haired, bespectacled boy who didn’t know he was a wizard became more and more real to me. I think that perhaps if I had had to slow down the ideas so that I could capture them on paper I might have stifled some of them (although sometimes I do wonder, idly, how much of what I imagined on that journey I had forgotten by the time I actually got my hands on a pen).