I'm trying to purchase a ticket from Orte to the train station in Rome on Trenitalia's website, but when I select my departure then click "continue" I get a dialog box that says, "We are sorry but the chosen solution is not purchasable". Does anyone know what that means? When I arrive at the train station will I be able to purchase a ticket then, or does this somehow indicate that the train is sold out?
It just means you cannot purchase it online, or, cannot purchase it at the time you are trying to do so.
Incidentally, you won't be able to purchase any tickets on the Trenitalia wbsite because they don't accept credit cards from US. There are many, many discussions about this topic on this site- just search on Trenitalia if you want more detailed information.
Most of the folks on this site say that you don't need to purchase in advance, and those who had trouble not getting a ticket when they got there stated that it was likely due to a holiday or other very busy train day. We just recently returned from two weeks in Italy and did not have any problems. Once we had to take a train one hour later than we wanted, but that was the only issue.
Nancy, most ot the train runs from Orte to Rome will be on Regionale trains. This is the local. No seat reservatons permitted. It's like your local bus at home. You buy your ticket, get on and find your own seat. If no seats, you can stand. Trenitalia most likely wouldn't sell you a Regionale ticket online in advance anyway. Parking your car in Orte and taking the train into Rome is a great idea. I see 45 trains running each weekday from Orte to Rome so don't worry about missing a train. 50 minutes and you are at Rome Termini.
For this journey, there is the occasional IC train (Intercity) that does require a seat reservation. You can easily buy this ticket in Italy. The seat assignment will be done by either the ticket agent or by the ticket machine should there be on in Orte and you choose to use it. You can also buy this ticket at any previous train station you visit prior to Orte.