Train travel December

Hard to forecast but I would not think so. There are lots of trains. About 53 between Florence and Rome. First class is always wide open. You can switch fairly easy if you have a base fare ticket. All trains except Regionale require a seat reservation so you cannot just jump on a train unless it is a regionale train. Don't know if either is better. They do not always use the same station so be sure the train goes to the station that you want. Go with schedule and price.

Seats never sell out entirely for any given train. At most they sell out in 2nd class (now called Standard on some routes), in which case you may have to buy a seat in business class (or 1st cl.) or purchase, if you aren't in a hurry, a ticket in the next train which almost likely have availability for 2nd class (standard). There are 1 to 3 trains per hour on that route. Sundays are busy days with weekend travelers, especially in the evenings, but not so much in December. Travel in late mornings or the middle of the day and you should be fine.

I took Italo round trip Rome/Naples/Rome a year ago: Italo was spotless, on time, comfortable,quiet,has high quality staff. I took it because of threatened strike on Trenitalia. I cannot comment on Trenitalia's Freccia service, which,like Italo, is high speed.