Location from harry-potter-philosophers-stone by
A hidden railway platform at King's Cross Station in London, accessed by walking through the barrier between platforms nine and ten, where the scarlet Hogwarts Express departs every September 1st at exactly eleven o'clock.
Platform Nine and Three-Quarters is the threshold between Harry's old life and his new one. He arrives at King's Cross alone, bewildered, with a ticket for a platform that doesn't appear to exist. The Dursleys dropped him off with undisguised relief. He stands between platforms nine and ten, surrounded by Muggles, with a trunk, an owl, and no idea what to do — until he hears a red-haired woman saying '...packed with Muggles, of course...' and follows the Weasley family through the barrier. Molly Weasley helps him through. Fred and George help him with his trunk. Ron sits with him on the train and shares his sandwiches. The Hogwarts Express departs at exactly eleven o'clock, and Harry watches London fall away behind him, heading toward a place he has never been but already feels like home.
A bustling platform wreathed in scarlet steam from the gleaming Hogwarts Express. Families in robes and Muggle clothes crowd the platform, loading trunks and owl cages. The scarlet steam engine idles at the platform edge, its compartments filling with students. The barrier from the Muggle side looks like a solid brick pillar between platforms nine and ten — you walk straight at it and pass through.
Also known as: Platform Nine and Three-Quarters, Platform 9 3/4, King's Cross, the platform