Platform nine and three-quarters is the train platform from which students board the Hogwarts Express, the scarlet steam engine that brings students to and from Hogwarts. It can be accessed by walking straight through the apparently solid barrier between platforms nine and ten.

In the Harry Potter universe, Platform 9 3/4 is the secret magical platform at King’s Cross Station in London where students board the Hogwarts Express every September 1st. The fraction "9 3/4" signifies its hidden location nestled between the Muggle platforms 9 and 10. To access it, witches and wizards must walk directly through the solid-looking brick barrier at a steady pace, usually with their luggage trolleys. J.K. Rowling has noted that the use of a fraction suggests a hidden layer of reality existing within the mundane world. At the real King's Cross Station in 2026, fans can visit a commemorative photo spot in the main concourse featuring a luggage trolley disappearing into a wall. Interestingly, Rowling later admitted she had confused the layout of Euston Station with King's Cross when writing the books, as the real platforms 9 and 10 at King's Cross are actually separated by tracks rather than a single brick wall.