Yes, you will have to change trains in Naples to go to both Pompeii and to Sorrento. We did this when we were in Italy last Fall and had no problem at all dealing with the railway station in Naples. The local train out of Naples(the Circumvesuviana) is on a lower level but it is well signed and you will find escalators and a moving walkway. Just be very careful to get on the train going to Sorrento. There are 2 lines which run parallel for part of the journey (Sorry I don't remember the town to which the other train was destined but I think it also began with S....)

Depending on how long you are staying in Sorrento, do consider going directly there from Naples and taking the train back from Sorrento to Pompeii another day. The journey is only about 20minutes and is quite inexpensive - the circumvesuviana is a local/tourist train and runs very regularly.

I say this because the trip from Rome to Naples and onwards can be quite tiring when you've got luggage to carry and I'm sure you would want to spend several hours walking around Pompeii. We'd really recommend a guide to get the most out of that experience.From our perspective, we would not want to miss Pompeii and would choose it over a day in Naples if the choice had to be made - but that's a very personal choice.

One other suggestion is that by chance we took the Eurostar from Rome to Naples, we took 1st class because we didn't want to risk no seats and this was just about the most comfortable train trip we've ever had.Apparantly Fridays and Sundays reservations are mandatory on the Eurostar. Hope some of this helps. Have a great trip Bob and Judy, Abbotsford BC Canada