He didn't even know her name, but the girl had come into his compartment, and sat upon his shrunken form, apparently not even knowing that he was there. He recalled the girl, whoever she was, wasn't all that bad looking; she had bushy brown hair, and she was already dressed in her Hogwarts robes, which for a girl, seemed to include a skirt. For now, he was starting to feel quite acquainted with that skirt, as he was buried under it, and through it, he could feel the momentous weight of her butt cheek bearing down upon him, owning him, seemingly without her being aware. From his position, he was trapped under a gigantic girl-mountain.
Above him, Hermione continued to read her book, oblivious to the fact that she was sitting on the tiny form of a shrunken boy.-who-lived.
After a while; Harry had no concept of time in his pitch dark prison of girl-flesh and skirt, his wand started to glow once more. What was the matter with his wand? It had got him into this mess, was it going to make matters even worse? Was that possible? Whatever it was doing, Harry was powerless to stop it, for he had never learned how to use his wand; it was acting of its own accord.
The glowing wand provided a certain amount of light, for Harry. It lit up his little tent for a second or so; the pest way to describe his place under Hermione' skirt. Harry could make out the cushion of the seat on which he rested, he could see the fabric of her skirt, and he thought for a moment, he could see the contours of a girl's butt cheek, he could certainly feel it crushing him under its young womanly weight.
Harry couldn't quite tell what the wand had done this time, aside from offering him a momentary view of his girl-prison. Then he noticed that her skirt seemed somehow to be less substantial, as though he might be able to pass through it. A few seconds later, Hermione adjusted herself in her seat, lifting just slightly, just long enough for the magic to pull Harry completely through Hermione's skirt, which then solidified (from his perspective) once more.
Now Harry found himself trapped inside, not under the girl's skirt, and almost immediately, she settled into place once more, with Harry now covered not by her skirt, but her knickers; her skirt was now under him.
Hermione was still unaware of Harry's presence beneath her, as she continued to read her book.
At one point, a woman came by, pushing a cart full of sweets. Hermione bought a selection for herself to enjoy as she continued to sit and read.
Harry noted that his new home, even closer to Hermione's butt was quite warm; perhaps her own body heat was spreading around him, he started to sweat, quite profusely. However, simply being sat upon by this girl was torture enough, Harry didn't consider sweating to be all that significant.
That was until, a few hours later, the train arrived at its destination and Hermione finally stood up, ready to depart. It was as she stood up that Harry noticed that he was somehow stuck to Hermione's knickers, even from a standing position. He couldn't free himself, and from his place, trapped between her knickers and his skirt, nobody was likely to notice him.
Hermione left the train, not knowing that she had spent most of the journey sitting on the shrunken body of the famous Harry Potter, who was even now plastered against her knickers.