If Uni is smart, they will let people get on the train in one park, then when they step off the train in the other park, have windows or great visibility of all the Harry Potter stuff beyond the train station, making people want to leave the train station and explore the rest of it. Then people will be eager and willing to upgrade their tickets on the spot. Think carrot not vinegar. The wait for the train ride back would be another incentive. Just think of the convo, "Oh, honey, look it's 30 minutes to get back on the train to head back, why don't we just go ahead and get the upgrade so we can see the rest?" Uh huh. Exactly.