This is insane.
Park A and Park B.
If you want to go from A>B or B>A you buy an A+B park ticket, park hopper, whatever you want to call it. With an A+B park ticket, you get on the first three carriages of the train via an A+B turnstyle and queue. When you leave the train at the park you have travelled to, you exit through a new set of turnstyles serving as the PARK ENTRANCE for the A+B park ticket.
If you are wanting to do a round trip, just to experience the train ride, you get a round trip ticket (I assume this will be free). You pick up your ticket from a ticket booth letting you know your ticket is for the round trip at _____ o'clock. You then enter the train via the round trip queue and turn styles, which lead you into the last three carriages of the train. THESE CARRIAGES DO NOT ALLOW YOU TO EXIT AT THE OTHER PARK. The doors of the carriage will not open, meaning no one can make the mistake of accidentally getting off. Instead, you wait until the next group people have boarded and take the train back to the park you originated from.
There may be separate trains for round trips and A+B park ticket holders, and there may be different configurations on train time tabling and carriage set up in order for the most efficiency of round trip ticket holders.
How they theme these, I don't know. It is not complicated how this system should work, or will work. Just because there is a train doesn't mean anything changes to how A+B park tickets work.