and plan to go on a Monday or Tuesday so there is no Halloween Horror night.
If you were to start at Universal Studios *park* first and get it all done before 5, you could go on a
HHN day and it wouldn't be a problem. It's just the one park that closes for the party.
Some way to take Harry's journey. My son, a red-head, was happy to take Ron's journey. (or any wizard from a wizarding family) He reasoned that any kid, especially a kid with older siblings like Ron had, from a wizarding family would have been to Hogsmeade before. It is a working, lived-in town, after all, so while Rowling didn't show us holidays where family came for visits, they might very well have had such events. So the families would have gone back and forth, and going to Diagon first is only something for those from non-wizarding families.
Of course, since we first went to Uni when there was ONLY Hogsmeade, and he had been there several times, it also went along with his experience.
That said, to get BOTH ways on the train, you need to start in park A then go back immediately to tour the Park B (and then take the train again to tour Park A) or start in A, tour around, go to B, tour around, go back to A to get both train rides.