Taking train from Rome > Orvieto next Friday, should I reserve? Use app? Or buy at station?

Most of the trains are regional with no reserve seating so there is little reason to buy a ticket in advance.

There are a couple of intercity that are maybe 15 min faster so I would stick with the Regional and buy it that day or the day before.

Be sure to validate (time- and date-stamp) your ticket not too long prior to getting on the train. There are machines in the station for that purpose.

Buy at the station or on the Trenitalia app when you go.
There is no wisdom in buying a Regionale train ticket ahead of time.

jk,

As the others have mentioned, most of the trains on that route are Regionale so no reservation is required or even possible. I typically have a look at the schedule on the Trenitalia website so that I'm aware of what type of train I'll be using, and can choose the best departure time for my schedule. If you decide that one of the Intercity trains works better for you, the seat reservation will be included with the ticket.

You can buy a ticket at Roma Termini either from the staffed ticket office or from one of the many kiosks (they have English language options and are very easy to use). If you buy Regionale tickets locally, you must validate prior to boarding the train on the day of travel, or risk hefty fines! If I'm in the vicinity of a station the day prior to my trip, I buy the ticket then (be sure to specify the correct date of travel). As I always travel with a mobile phone these days, the Trenitalia app is also an option.

When you arrive in Orvieto, you'll have to take the Funicular from the station up to the town.

Use the app, no need to print out and time-stamp e-tickets. As long as you have a working smartphone, of course.

You will have 4 hours to get to Orvieto using any Regionale on that route, the only downside of e-tickets is that the countdown begins from the departure time of the train you have chosen on the app.

Dario, what does that mean about the 4 hour countdown?

jgg,

As Dario mentioned, electronic tickets purchased through the App do not have to be validated. However they do have a "shelf life" and the passenger must depart within four hours of the departure time they chose when they bought the ticket. As I understand it, if the four hours expires while they're still in transit, there will be no penalty or fines.