The Hogwarts Express was the large, gleaming scarlet steam engine that carried students from King's Cross Station in London to Hogwarts School of Witchcraft and Wizardry in the Highlands of Scotland.
Prior to the imposition of the International Statute of Secrecy, wizards had been using a wide variety of methods to reach Hogwarts on 1 September annually. Some rode brooms, some drove carts and carriages, some rode a variety of magical creatures, and yet others tried to Apparate into the school, and failed because of the Anti-Apparition Charms protecting the place.
When the Statute of Secrecy came into effect, it soon became urgent to find a secretive, unobtrusive manner of carrying the hundreds of students to the school annually. At first, Portkeys were set up all over the country, which caused more problems than they solved; students would miss their time-slots, be unable to find their Portkeys or else end up in the hospital wing for the first week of school due to Portkey-sickness.
Headmasters refused to allow the authorisation of the Floo Network into the school, and the problem continued with no solutions being suggested; that is, until Ottaline Gambol, who had been fascinated by Muggle technology, recommended a train.
It has never been definitively proven how the Hogwarts Express came into existence, but there are secret records at the British Ministry of Magic detailing an operation involving a hundred and sixty-three Memory Charms and the largest ever mass Concealment Charm ever performed in Britain. The morning following these supposed services, a gleaming scarlet steam engine and carriages shocked the villagers of Hogsmeade, who also had failed to realise prior to that point that they had had a train station, while a number of confused Muggle railway workers in Crewe spent the year worrying over the feeling they had misplaced something valuable.
The Hogwarts Express has undergone several magical modifications before it was deemed suitable for school usage; many pure-blood families were outraged at the prospect of their children riding a Muggle method of transport to school, calling it insanitary and unsafe, but as the Ministry told them that students either rode the train or did not go to school, their objections were quickly quietened.