When rides get stuck (In the message box it will say "Car/boat something hasn't returned to station yet") which mostly happens on boat hires or Wooden Crazy Rodent Roller Coasters, just pause the game, click on the the stuck ride, and double click the red light. The ride's boats/cars are now gone. Re-open the ride and then un-pause the game. The Cars/boats are no longer stuck and your line is still there (Although there is a mob like crowd of people, as you will notice once you colse the ride).