Instead of being at the train station saying bye to the kids, Harry is in the basement of Kings Cross where Professor Dumbledore stood waiting for Harry to say bye to him. And P.Dumbledore kissed Harry on the forehead.
Harry said: "Come Professor, join us!"
in which he replied, "I cannot! I have important things to do"
So Harry went back upstairs to join the crew and then after the Hogwarts Express left, they all went wild and got in touch with their inner child and flew everywhere on their broomsticks and played Quidditch around Kings X, which turned into the Dark Forest which didn't look so dark.