You have to get off at Caribbean Beach and switch to a different car. The trip itself does not take long (we went from Epcot to HS last week in 15-20 minutes total) but the cars do occasionally stop while you are enroute so allow some extra time.
It should be under 10 minutes total, if there's no line at the Caribbean Beach transfer station. It's about 2-3 minutes from Pop to CBR, and another 2-3 minutes from CBR to Riviera.