The Journey From Platform Nine and Three-Quarters

Book 1, Chapter 6.

"People jostled him on their way to platforms nine and ten. Harry walked more quickly. He was going to smash right into that barrier...he broke into a heavy run...he closed his eyes ready for the crash--
It didn't come...he kept on running...he opened his eyes.
A scarlet steam engine was waiting next to a platform packed with people. A sign overhead said Hogwarts Express, eleven o'clock. Harry looked behind him and saw a wrought-iron archway where the barrier had been, with the words Platform Nine and Three-Quarters on it. He had done it."