Location from harry-potter-order-of-the-phoenix by
The hidden railway platform at King's Cross Station -- this year notable for Sirius's reckless appearance in his Animagus form to see Harry off, and the growing paranoia about Ministry surveillance.
Sirius comes to the platform in his dog form to say goodbye to Harry, and Lucius Malfoy recognizes him -- or suspects enough to use the information. It's a small reckless act that captures everything about Sirius this year: the need to be present, the refusal to stay hidden, the willingness to risk everything for a moment of connection with his godson. The platform is also where Harry first noticed the Thestrals pulling the carriages at the end of last year. The world looks different now. The steam from the Hogwarts Express is the same scarlet it's always been, but Harry is not the same boy who first walked through the barrier at eleven.
A bustling platform wreathed in scarlet steam from the Hogwarts Express. Families in robes and Muggle clothes crowd the platform, loading trunks and owl cages. The barrier from the Muggle side looks like a solid brick pillar between platforms nine and ten.
Also known as: Platform Nine and Three-Quarters, Platform 9 3/4, King's Cross, the platform