Had a wonderful trip and positive train experiences in Italy. Before departing, I read Rick's post https://www.ricksteves.com/travel-tips/transportation/trains/should-i-get-a-rail-pass. Given that I was traveling longer distances in Italy on three of the days, I opted for the 3-day pass and advance booking which worked out quite well.
Venice to La Spezia-> Riomaggiore was stellar (switched trains in Firenze and chose the somewhat slower/direct Regionale as opposed to changing trains again in Pisa). Turned out to be a good move- Pisa was quite crowded and unless we ended up directly in front of the doors when the train stopped, we wouldn't have gotten a seat.
On the Frecciarossa from La Spezia to Rome, there was a medical emergency ~1/2 of the way there. The train made an unscheduled stop and medical personnel were dispatched. Upon arrival, they evaluated and removed a passenger. We missed our connecting train at Roma Termini bound for Naples. The Rome Termini station was something along the lines of a whirlpool of people in a semi-disoragnized state of constant motion with long queues all over (including in front of the last minute train assistance booths.
I decided we were hopping on the next "4-digit" fast train to Naples without missing 2-3 trains while waiting in a long line to get a reissued ticket. We ended up standing in between two cars with 5 semi-behaved hoodlums who were pushing and shoving each other and smoking the whole way to Naples. They were sizing me up for sure though I think they sensed my willingness for "international relations encouragement" and decided to stay out of my grill.
No one came along checking train tickets so I guess we lucked out.
The Circumvesuviana was even more intriguing. That is the first time I've encountered panhandlers breathing down my neck at the ticket window asking for a handout whilst I was purchasing tickets. The trains had been running very late all day so the platform was packed with people (3/4th's of them were smoking like stacks- doesn't particualrly bother me; my wife was a bit put off). We eventually boarded the severely overdue train, standing room only.
Teens were punching, slapping and hitting each other, throwing empty water bottles around, etc. There were a few adults on board who appeared to be looking for "an opportunity". I remained standing next to our small amount of luggage for a good bit of the 1+ hour ride to Sorrento. I've been on worse subways and trains (not many though).
Next time, I might take the train from middle/Northern Italy to Salerno instead and then a bus or ferry to Positano or Amalfi.