Fiction by Eva Ibbotson.
Although this is a children's book that features a railway platform at King's Cross Station in London that leads to a magical world... it's NOT related to the Harry Potter series of books. As far as I can tell, this author and J.K. Rowling came up with similar ideas independently. Or else there's actually something magical about King's Cross Station...
This is a super-fun fairy-tale story about a lost prince from a magical world, and it's not just for kids.
I enjoyed it a lot; it reminded me of The Princess Bride or Ella Enchanted a bit. Someone should make a movie!