What platform number do students use to board the Hogwarts Express?

The story behind the answer

Students board the Hogwarts Express from Platform Nine and Three-Quarters at King’s Cross Station. It’s a concealed platform accessed by walking straight through the barrier between platforms 9 and 10, first shown when Harry follows the Weasley family in his first year.

Fun context: J.K. Rowling set the scene at London’s King’s Cross, and the films used nearby platforms