I think the whole idea of this train is to unify the resort though...
I mean how many people now JUST go to one park? That number will lessen significantly with this addition. I believe the 1-park pass will become all but obsolete, as its barely even advertised as it is now. People will want the full Harry Potter experience, and considering a 2-park ticket is a better value anyway, I dont think queue lines will be short simply due to lack of interest in park-hopping.
Not to mention the fact that it is one of only a few non-restrictive rides at the resort will ADD to its popularity.