The Hogwarts Express is the crimson and gold train that delivers students to Hogwarts. It leaves from Platform 9 3/4 at King's Cross station in London, England.
The Hogwarts Express is noted for having a nice old lady with a trolley full of sweets who visits each compartment to sell her goodies.
It is also noted for being a common early setting in Harry Potter fanfic, as the restricted space is very convenient for writers who want their OC to wind up sitting with the canon characters in the last available train compartment.