Platform 9 3/4 is probably the most famous train platform in the world. However the platform is fictional, created by J. K. Rowling.
The platform was mentioned in the first movie, when Harry, the Weasel family and lots of other wizard kids had to travel to Hogwarts by train. The train could only be accessed from a platform that couldn’t be seen from the normal platform so the kids needed to run through the wall with their trolleys.
The platform 9 3/4 is supposed to be between the 9th and 10th platform, but due to high traffic the authorities decided to place the sign just in the main hall of King’s Cross.