Kind of new around here so forgive me for going back a bit and bringing up the discussion of folks being able to ride who just have one park tickets. Now, I have no inside information and really I know very little about Harry Potter, but for some reason I wanted to mention how I would handle this.
From reading here, I am under the impression that the whole train is an illusion. Couldn't the train have different destinations? Say a line for single park ticket holders that did not travel to the other park? I would think they could have two train cars that where at the end of each track that never actually left the station, they would be stationary cars where the whole experience was simulated let you never really went anywhere so when you departed, you would still be in the same park.
Just a thought on how I think it could be done.