On the train station platform they have dancers, music, and the Mayor comes out to welcome everyone to the Magic Kingdom. Then the train comes with all the characters waving. Mickey also welcomes everyone, then a pre-selected family gets to countdown to opening and throw the pixie dust to open the park. (That's probably not a very good description, lol, but it's hard to describe, at least for me!) It was very nice and we all enjoyed it. It made a wonderful and special start to our first day at Disney World. I'm not sure of the length, maybe between five and ten minutes. You do need to be there early as it is before the opening of the park. We got there about 20 minutes before park opening, but it wasn't very crowded at that time of year.
Hopefully someone who's better at descriptions will come along and elaborate!