"Well, there you are, boy. Platform nine -- platform ten. Your platform should be somewhere in the middle, but they don't seem to have built it yet, do they?"
-- Vernon Dursley, PS6
In northwestern London lies Kings Cross Station, a Muggle railway station that is one of the busiest in the city. It’s a perfectly ordinary station, with big plastic numbers over each track and guards who have never heard of Hogwarts School (PS6). However, every September 1, as the clock overhead approaches eleven o’clock (CS5) and the InterCity 125 pulls into platform nine (PA5), a strange crowd turns the occasional Muggle head. The crowd is wizarding children, bearing enormous trunks and caged owls, making their way toward – and then through – the solid metal barrier between platforms nine and ten (OP10).
Through this barrier, of course, lies Platform nine and three-quarters, home of the famous Hogwarts Express. There a look back at the barrier reveals it is instead a wrought-iron archway with the words “platform nine and three-quarters,” and over the scarlet steam train billowing smoke, a sign announces that the Hogwarts Express departs at eleven o’clock (PS6). The platform also has a distinct smell – one which lifts the spirits of young wizards who, like Harry, look forward each summer to the start of term (OP10).
Though the platform is normally only used six days a year (round trips for the beginning and end of term, as well as the Christmas and Easter holidays), the platform on those days is awash with activity. Steam from the scarlet engine floods the platform as cats wind their way around the wizards’ legs and owls hoot to each other (PS6), as hoardes of students and parents move around through the steam “like dark ghosts” (GF11), their voices carrying through the mist (DH/e).
Finally, at 11:00, a whistle sounds announcing the train’s departure (PS6), and the hiss of the pistons fills the platform as the train begins to move (GF11), creaking out of the station as family members stand on the platform and wave their children off to school (OP10). Nobody returns to the platform, then, for months – until the end of term, when it once again fills with wizards and a wizened old guard sits by the archway back to the Muggle world, making sure everyone gets home without attracting too much attention (PS17).
After being hit with Voldemort’s Killing Curse in the Forbidden Forest, Harry Potter found himself in a place that vaguely resembled Kings Cross – a “limbo between life and death” (BLC) – where he met and talked with Albus Dumbledore. The Kings Cross that Harry encountered here was, as he described, “a lot cleaner and empty,” and with no trains. As Dumbledore explained, the encounter “of course” took place entirely inside Harry’s head – “but why on earth should that mean that it is not real?” (DH35)
A trip to the real Kings Cross Station reveals that platforms nine and ten that do not closely match the description in the books. J.K. Rowling has explained the error:
“I wrote Platform 9¾ when I was living in Manchester, and I was actually thinking of Euston. So anyone who’s been to the real Platforms 9 and 10 at King’s Cross will realise they don’t bear a great resemblance to the platforms nine and ten as described in the book, and that would be because I was thinking of Euston at the time (HPM).”
The platform actually used for filming at King’s Cross Station in the movies is Platform 4. Hagrid and Harry are also filmed walking over the pedestrian bridge across the tracks.
From the Web
Platform Nine and Three-Quarters original writing by J.K. Rowling on Pottermore
Platform Nine and Three-Quarters from Harry Potter Wiki
How does one leave platform nine and three-quarters? from Scifi Stackexchange