From a PR perspective, I'm surprised they didn't go the other way with the most recent price increase and get rid of the 2-4 day single park per day tickets. Leaving 1 day, 1 park and 1-4 day park to park tickets. Advertising it as park to park access in included in all multiday tickets. I still think the best Disney comparison to the Hogwarts express is the Monorail. In Orlando, you can ride the monorail to you're hearts content with no ticket required. It's "FREE". In Anaheim, the exit to the monorail is in the middle of Tomorrowland. Before you board the monorail you need to present a valid ticket for the park the exit is located at (
Disneyland). You are not allowed to board at DTD, ride all the way around back to DTD without a valid ticket. Likewise at Universal. The exit to Hogwarts Express - Kings Cross Station is the the middle of Hogsmead at Islands of Adventure. You need a ticket with valid admission to IOA to board. Likewise, exit to Hogwarts Express - Hogsmead is in the middle of Universal Studios Florida. Since you've already been in the other themepark before you board, a Park to Park ticket is the only one that works.