Train travel advice in Italy

I don't know how to say it without sounding rude, but your itinerary is not manageable. You'll spend too much of your waking time in transit to have enough left over to sightsee. You are planning to go to several places that are not served by fast trains and you'll use up the better part of a day getting there, leaving you little, if any, time to sightsee before the places close.

I suggest you start by using trenitalia.com to see train schedules and prices.

Sorrento The only train is the Circumvesuviani local train that takes an hour or more from Naples and runs twice an hour.
Orvieto and Assisi train stations are at the bottom of the hills that the the old towns rest on. From the stations you have to take a bus or funicular (Orvieto) into the old town.

Even if by "one day" you mean two nights with a day in between I don't think you've allowed enough time in many or all of the cities, without considering the travel time between cities. Surely Venice and Rome each need more than one day/two nights.

I would agree with the others: with this itinerary, you're going to be packing, unpacking, checking in and out of hotels, and seeing the inside of a train but not much else. I would cut Assisi and Siena, add those days to Rome, plus do Orvieto as a day trip from there instead of physically moving hotels.

In addition to Trentalia, check out the Italo website. Much more user friendly website, nice trains, nice service, and separate wait area in train stations. Overall, I think a better experience than Trentalia trains.

Don't mean to rain on your parade, but it appears you are trying to do way too many places in way too short a time frame. Please rethink you itinerary so that you can enjoy your trip rather than just enduring it.

Thank you - I really appreciate all the input! I should have explained - we have been to both Venice and Rome, so we were ok with passing through both of them quickly (we just have to fly into Venice). I actually used Rick Steve's Italy 2015 book, pgs 6-8, and kind of reversed and modified his 22 day trip to 18 days. I wish I could upload it here to show everyone, but basically it is (sleeping towns listed): Day 1/Milan; 2-3/Varenna; 4-5/Bolzano;6-7/Venice; 8-9/Vernazza; 10-11/Florence; 12-13/Sienna; 14/Assisi; 15/Orvieto; 16-17-18/Sorrento; 19-20-21/Rome. I thought it looked a little rushed, but was depending on Rick's expertise! We wanted to spend the most time near Sorrento, in Florence, and in Cinque Terre, and were considering everything else a "bonus" along the way. But I am very interested in hearing opinions - thank you!

OK, The RS "Best of Italy in 17 Days Tour" is similar, minus Sorrento. Remember, they are using a charter tour bus and can blitz through things a bit more efficiently.
What are your flight arrival and departure times and dates? You've got the flights booked, assuming into Venice and out of Milan. So the beginning and end points are set. You've said Sorrento, Florence, and Cinque Terre are the "must dos". Now its time to edit that down a bit.

You said you have hotels booked, so making itinerary changes would require a lot if re-booking. Too late to get big discounts on train fares, so you could just buy train tickets at the station for maximum flexibility.
Venice to Sorrento looks like the longest trip, so you might want to lock in seat reservations (to Naples -- Circumvesuviana to/from Sorrento is non-reserved, like a city bus; just push your way on and try to get a seat. Or fly Venice to Naples and take a less-crowded bus from Naples airport to Sorrento. )

Rick's itinerary is laid out specifically for train and bus travel and is doable. But good practice for you to fully picture it is to lay out the plan on a calendar and pencil in travel time for each route, either from the Connections summary at the end of each of Rick's chapters, or from the real train schedules. Are you early birds? If Rick had a 9:00 a.m. train in the back of his mind and you catch the 11:00, those two morning hours are probably "lost," not spent on sightseeing. Not to say that morning is the only time to travel; some people might take a train during the afternoon pause when some sightseeing is closed, or take a picnic dinner onto a later train. Note that there is only one daily direct bus between S. Maria degli Angeli (down the hill from Assisi) and Siena, going toward Siena in morning and toward Assisi in evening: http://autolinee.baltour.it/Home/455-1-en.html.

Appreciate all the input - thank you!