Many travelers can get by with just a carry-on size bag regardless of location and weather. You will find train travel a lot easier if have only one carry-on and perhaps a shoulder bag.
You might take a look at the site structure. Most questions concerning travel are placed under the Q & A section and in the specific country. Just read the short description for each section. For example -------
Destination Q&A
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The boats are seasonal and will probably stop in October. Still have to get all the luggage to the boat and to your hotel. The Circum...... is commuter train with open seating. Put your bags where ever it is convenient but not on another seat. There is no special place for luggage.
How are you getting to Naples?