Going from Naples to Rome-which train is advised? Trenitalia Frecciarossa or Italo? Is the ferry stable (in good weather) or might someone with motion sickness be affected?
Two different companies, Trenitalia is state owned, Italo is private and only runs high speed long distance rail I have found at a lower price than Trenitalia's Frecciarosa high speed train product. I have only experienced strike threats and gross delays that screwed up my plans taking Trenitalia, never taking Italo.
Take whatever trains that meets you schedule and price. Trenitlia is the Italy National railroad. Frecclarossa refers to the type of train -- high speed. Italo is a private train using Trenitlia track system. Italo is high speed and far less frequent. There is little difference between the two train group. We have used Trenitlia for thirty years with no problems.