Purchased our tickets as soon as they became available Rome -> Florence. All 18 of us were reserved in Carriage #4. Guess what carriage was NOT connected to the train??? Yep - Carriage #4! Of course Italian Train personnel on the train and on the platform have no idea as to the answer, and as soon as it gets too complicated they all just completely disappear.
Lesson learned on this trip: If your ticket has you on a Carriage that doesn't exist on your train - head for the nearest 1st Class Carriage and take the open seats! Conductor came by later and told us he had found seats for us in the very last car on the train. We said it would be too difficult to drag all of our luggage through the middle of the moving train. He shrugged and went away and we rode 1st class.