King's Cross Station is a big train station in London. Besides the normal, Muggle train platforms, there are a number of hidden platforms accessible only to wizards. These platforms can be accessed by walking into the barriers between two platforms. Platform Nine and Three-Quarters is the platform from which the Hogwarts Express, the train which takes students to Hogwarts, departs.
From the Story
"The Hogwarts Express departs from King’s Cross station in London, taking students to Hogwarts School of Witchcraft and Wizardry. Students magically access platform nine and three-quarters through the barrier between platforms nine and ten."
New from J. K. Rowling
"When Ottaline Gambol commandeered a Muggle train to serve as the new mode of transport for Hogwarts students, she also had constructed a small station in the wizarding village of Hogsmeade: a necessary adjunct to the train. The Ministry of Magic felt strongly, however, that to construct an additional wizarding station in the middle of London would stretch even the Muggles' notorious determination not to notice magic when it was exploding in front of their faces.
"It was Evangeline Orpington, Minister from 1849-1855, who hit upon the solution of adding a concealed platform at the newly (Muggle) built King's Cross station, which would be accessible only to witches and wizards. On the whole, this has worked well, although there have been minor problems over the ensuing years, such as witches and wizards who have dropped suitcases full of biting spellbooks or newt spleens all over the polished station floor, or else disappeared through the solid barrier a little too loudly. There are usually a number of plain-clothed Ministry of Magic employees on hand to deal with any inconvenient Muggle memories that may need altering at the start and end of each Hogwarts term."
J. K. Rowling's Thoughts
"King's Cross, which is one of London's main railway stations, has a very personal significance for me, because my parents met on a train to Scotland which departed from King’s Cross station. For this reason, and because it has such an evocative and symbolic name, and because it is actually the right station to leave from if you were heading to Caledonia, I never knew the slightest indecision about the location of the portal that would take Harry to Hogwarts, or the means of transport that would take him there.
"It is said (though where the story originated I could not tell you; it is suspiciously vague) that King's Cross station was built either on the site of Boudicca's last battle (Boudicca was an ancient British queen who led a rebellion against the Romans) or on the site of her tomb. Legend has it that her grave is situated somewhere in the region of platforms eight to ten. I did not know this when I gave the wizards' platform its number. King’s Cross station takes its name from a now-demolished monument to King George IV.
"There is a real trolley stuck halfway out of a wall in King's Cross now, and it makes me beam proudly every time I pass...."