If we are ending our car rental in Tuscany, specifically Pienza, & then need to catch a train to the Amalfi Coast, specifically Sorrento, which town would be better to drop off our car & then catch the train to Naples to continue on to Sorrento? Chiusi or Orvieto?
Stephanie - Really, whichever works best for you. To get to Sorrento by train from either town you'll need to go first to Rome. Not only do all roads lead to Rome but so do all train lines. From Rome you hop one the next train to Naples - there are many.
From Naples you'll catch a local commuter train called the Circumvesuviana to Sorrento.
Orvieto is somewhat farther south and that might be a slight advantage for the car drop off; but I'd think it depends more on where you're going to be the night before you drop off the car.
What rental agency are you using for your car? We just returned from Italy and picked up our car in LaSpezia and dropped it off in Orvieto. We rented from Autoeurope. If you are renting from them, go to their website to see what towns they have agencies in then chose the one closest to where you will be.
Good luck!