Places
Platform 9¾
Platform 9¾ is the hidden platform at King’s Cross from which the Hogwarts Express leaves. A witch or wizard reaches it by walking through the solid barrier between platforms nine and ten. The epilogue returns to the same barrier nineteen years after the battle.
First mention
Harry Potter and the Philosopher's StoneChapter: The Journey from Platform Nine and Three-Quarters
Molly Weasley shows Harry how to pass the barrier, and he pushes his trolley through onto the scarlet train.