Platform Nine and Three-Quarters
Platform 9-3/4 exists in magical space within King's Cross Station, London. Assuming that the Hogwarts Express uses existing Muggle rail lines, in the same way that the Knight Bus uses existing Muggle roads, the platform would be situated between Muggle platforms 9 and 10, sharing the track with those platforms. We know that no other trains were scheduled at 11 am, since Harry asked and was told there was no train departing at that time, so the track would have been clear for magical use.
This map is superimposed over a British Rail map of King's Cross Station. (full map)