The ride on the Hogwarts Express starts from King's Cross railway station platform 9 3/4, which is invisible to Muggle eyes and is reached by walking through the barrier between platforms 9 and 10.
Site: Platform 9 3/4 (1 memorial)
N1, Euston Road, King's Cross Station
We were lucky to have the camera ready just as a pupil pushed his luggage trolley through the wall on his way to the Hogwart's Express.
When the station was renovated this feature was moved, not far, to the new ticket concourse. We've moved our pin accordingly but have kept the old photos.