Location from harry-potter-prisoner-of-azkaban by
The hidden railway platform at King's Cross Station where the Hogwarts Express departs, and where the journey to a year of Dementors, escaped prisoners, and time travel begins.
Platform Nine and Three-Quarters is a threshold. This year the platform buzzes with nervous energy — Sirius Black's escape is the only topic of conversation, and Ministry officials are visibly present, scanning the crowd. Parents hold their children a little tighter. The Daily Prophet has been running Black's photograph for weeks, and everyone knows the Dementors will be searching the Hogwarts Express. The train journey this year is interrupted when the Dementors board, searching for Black. The train stops, the lights die, the temperature drops, and Harry has his first encounter with the creatures — fainting, hearing his mother's screams, waking to find Lupin pressing chocolate into his hand. It's the first sign that this year will be different from anything that came before.
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. A wrought-iron archway marks the platform entrance on the magical side.
Also known as: Platform Nine and Three-Quarters, Platform 9 3/4, King's Cross, the platform