We are in Orvieto, leaving for Rome on Saturday. I bought our train tickets (only 10 Euros each!) today. It looks like we can either change trains in Tiburnia or Termini, depending on which train from Orvieto that we take. Correct?
I think you'll have an easier time of it at Tiburtina. Should be a train to Trastavere every half hour on Saturday. Its a modern station with an escalator up to the cross-over and an escalator down to your connecting platform.
Edit. If you take the IC train at 7:30, its nonstop to Termini.
IC requires specific ticket, make sure what type of ticket you have. IC will have a reserved carrozza and posti number on it, a regionale ticket will have the "Convalidare" message at the top.
You are correct. We are guessing you have a Regionale train tickets so make absolutely certain that you validate (time stamp) the ticket and you only get on a Regionale train. If you don't, the fine is substantial and paid on the spot. Ignore the comment about the IC train if you have a Regionale ticket. But, if by some chance you do not have a Regionale train ticket, then your ticket is only good for a specific train, date and time.
I guess it maybe depends where you are going in Trastevere, but if you are going to near the river, then just getting off at Termini and taking the bus may be quicker than changing trains...going to Trastevere Station, then backtracking on the tram or bus to where you want to go. The "H" bus will take you from Termini, across the river and down via Trastevere.