The Hogwarts Express uses Platform 9¾ in J.K. Rowling’s Harry Potter series.
The concealed platform is located at London’s King’s Cross railway station. Young witches and wizards reach it by passing through the barrier between platforms 9 and 10, then board the scarlet steam train for Hogwarts at the beginning of the school year. The platform number is one of the series’ best-known pieces of magical world-building.
In the novels, the train generally departs on 1 September. Harry first learns how to find the platform after asking the Weasley family for help at King’s Cross. The station therefore becomes an important gateway between the ordinary Muggle world and the wizarding world.
King’s Cross later installed a Platform 9¾ attraction and shop for visitors. The real station’s platforms 9 and 10 were redesigned during railway works, so the fictional barrier is represented separately from the exact book setting. The fraction is three-quarters, not a decimal or a conventional platform number.