Readers are reminded that Eurostar (ES), Eurostar-Fast (ES-Fast), and Alta Velocita' (AV) trains DO NOT stop at Orvieto, because Orvieto is off of the high-speed rail line between Rome and Florence.
The only trains which do stop at Orvieto are "Regionale" ("R"), "Intercity" ("IC") and a couple of night trains. The "R" trains are very useful to reach Rome and Florence cheaply (see following posting), while the IC trains go as far south as Naples and as far north as Venice and Milan.
It's usually better to take one of the direct IC trains if your destination is Naples, Bologna, Venice or Milan, rather than connecting to one of the faster AV or ES trains in Rome or Florence. The IC trains are considerably cheaper than the ES or AV trains, yet travel time is only slightly longer (in some cases, the same). While it's true that connecting in Rome to an AV train to Naples may save as much as one hour total travel time, it comes at a very steep cost indeed, compared to taking the direct IC train.
The IC trains typically have eight second class and two first class railcars per convoy, and no restaurant or cafe car. Second class is usually fine for just about everybody, although first class is a good alternative during the peak travel season, when the train is apt to be full. Some of the IC trains may have a rolling cart refreshment service with beverages and light snack items (but don't count on it). All seats on the IC trains are reserved, so there's no chance of having to stand. By contrast, no seats on the "R" trains can be reserved. All IC trains, and most "R" trains, have air conditioning.