It's something that I'm not sure if it was put in the books or was left outside the book for people to guess but... In any moment of the books, during the travel from the train between King Cross and Hogwarts, did they ask or checked the ticket? I remember the scene on first year/book that Hagrid almost forgot to give Harry the ticket, even when he did not tell about the 9 3/4, but I'm not sure if it was mentioned if they really need the ticket.
Yes, on at least one occasion there's a "ticket inspector" on the Muggle side of the platform barrier.
When the ticket inspector signalled to Harry, Ron and Hermione that it was safe to walk through the magical barrier between platforms nine and ten, however, he found a surprise awaiting him on the other side: a group of people standing there to greet him who he had not expected at all.
Harry Potter and the Order of the Phoenix
One assumes that a ticket inspector would be inspecting tickets.