Trains from Sorrento to Florence on a Sunday?

Let me start this story by saying that high-speed, direct trains leave from Naples to Rome at least hourly, up to three times an hour during the morning and evening rush, regardless of the day.

From Positano to Florence could be quite a long haul if you don't get tickets for the fast train from Naples.

We went only to Rome a couple years back, in the second weekend of April. We caught the first bus (posted for 7:50, but 13 minutes late) to Sorrento, then a quick transfer to the Circumvesuviana and we were on our way to the Naples station,... so far so good.

Because I wasn't sure we would be able to get to the Naples station in time to catch the 10:00 train, I did not buy tickets in advance. This turned out to be a huge mistake. All the direct trains--they take about 70 min. on average--were sold out until 8:30 that evening.

We got on the local and it left the station at just a few minutes before 11. By the time will pulled into Termini Station, it was well after 3:30. We got into a taxi with perfect timing to hit gridlocked, rush-hour traffic. By the time we got to our apartment in Trastevere, and settled in enough to try and make a sightseeing plan, street lights were coming on and it was nearly dark.

So yes, start close enough to the Naples Centrale station so you can confidently arrive in time to catch a train for which you've bought tickets 120 days in advance.

Kim, you'll be traveling with your baggage, and you might just find that getting a private car or taxi from Positano to the Naples train station is a worthwhile splurge. Your hotel could probably arrange something for you. Better than worrying about connecting to the Circumvesuviana and moving through the Naples Station quickly, or than going through a whole move just to spend the night in Sorrento (not to say there's anything wrong with spending a Saturday night in Sorrento, with a bottle of fine local limoncello!). When I myself made the trip from Sorrento to Naples to connect with a train going north, last May, I found that the long escalators leading up from the Circumvesuviana to the main Napoli Centrale were not working, leaving me a long and somewhat strained trek up.

Bus timetable Amalfi-Positano-Sorrento. There are plenty of buses on Sundays starting from 6:20am and about one bus every hour thereafter:
http://www.sitasudtrasporti.it/archivio/Download/corse/sitasud/Campania/Salerno/bac6d4a7-6670-45f5-8626-c4e341a34939_5070.pdf/0

Timetable for Circumvesuviana trains from Sorrento to Napoli P. Garibaldi. Sunday's trains are just as frequent. Basically at least two trains every hour.
http://www.eavsrl.it/web/sites/default/files/ftp_orari_ferroviario/Cy.pdf

For the timetable of fast trains from Naples Centrale to Florence SMN use:
www.trenitalia.com
There is no difference in the number of high speed trains between Sunday and other days. Basically at least one every hour all day from early morn until the evening, with no change required all the way to Florence (less than 3 hour trip).

The chances of having trains sold out is very close to zero. I just checked the timetable for this morning (it's about 6am in Italy) and no trains are sold out. Zero. In my whole life I have never experienced a sold out train. At most the 2nd class was sold out and I either waited for the next train 30 min later or paid for 1st class. In 30 years in Italy and over 40 times visiting from here I think I've taken more trains than most people who write on this board.

The only advantage of pre-purchasing is that you can get a discounted ticket (they sell fast). However discounted tickets are non changeable and non refundable, therefore if you change plans or you miss the specific train you book, your money is lost.

Larry's suggestion to take a private car (limo service) from Positano to Naples is not a bad ides. It's not cheap (€80 and up) but if you have a lot of luggage, it's worth it, so you don't have to haul it on the bus and the Circumvesuviana train. You save time too.

No need to change hotels the night before just to make the trip. I've left Positano in the afternoon even and made it home in Florence by dinner. And that was before the had the high speed rail finished between Naples and Rome.

Since no one else has mentioned it, unless things have changed you also have the option of taking the Italo train from Naples to Florence (through Rome). We did this last May when they were running a special with a 5 Euro upgrade from coach to Smart XL seating (essentially first class seats and space without the other first class amentites).