a VirtualTourist member
We would like to take a day trip from Rome to Pompeii. If we take the high speed train to Naples and then to Pompeii, should we purchase our tickets stateside or in Rome?
You can buy the rome-naples ticket on internet (www.trenitalia.com).
To go to Pompei I suggest you to use "Ferrovia Circumvesuviana": its station is very close to the archaeological site, while Trenitalia's station is far away.
In Naples the Trenitalia and Circumvesuviana stations are side by side.
More info here: http://www.vesuviana.it/web/en
As you have probably noticed, the faster trains from Rome to Naples all require reservations. However, there are so many trains making this run every day (www.trenitalia.com - look for English at the top), that there is no way that they all sell out. Plus, depending on your requirements, you may not want to take the fastest trains because they are the most expensive...go see the website, pick a day in the next week that is the same day of the week as when you are planning to go, and see your choices. Note: the AV, Eurostar, and IC trains all require reservations. The regional (R) trains don't take reservations at all (you have a ticket, you can get on).
Since the FS (rail system) will sell you a ticket at the last minute for a reservation-only train as long as there is a free seat, I would suggest waiting until you get to Rome and when you are sure which train you want to take. Reserving a train in this situation weeks in advance may actually be more hassle than it's worth, depending on how your plans might change once you're in town. Note: if you do buy ahead, there is a 15% discount on fast trains for tickets bought at least 7 days in advance (note: limited number of seats), and a 30% discount on fast trains for tickets bought at least 15 days in advance (note: limited number of seats). Also note that North Americans often report difficulties in buying tickets online from trenitalia.
Then, as gmg says, when you arrive in Naples, go downstairs to the Circumvesuviana station, buy your ticket there, and go. The Circumvesuviana is a commuter line and I don't even know if you can buy tickets in advance anyway...