Trai lines in Italy

How hard is it to travel by train in Italy?

Pretty easy. Trenitalia is the national carrier. Italo Treno only runs high speed trains between major cities.
Switzerland is very easy with at least one train in either direction every hour from all stations. Poland should also be easy, but I have not been there.

No, not all companies run out of all stations, but with few exceptions Trenitalia, the national railway https://www.trenitalia.com/en.html does run out of most all stations. You can use their app for most every journey.

The only other major railway in Italy is Italo https://www.italotreno.com/en They offer high speed rail on select routes, but do cover most of the very major cities. They ma offer better pricing, or a schedule that fits your needs better.

If in a search you see others that look like "Italian Train Companies" they are not, they are third party ticket sellers, they may be just fine, but why deal with a middleman, unless they can provide some value. Some examples would be "Italiatren" and "Italiarail"

There are however some private rail companies, usually running on separate lines, like the Cicumvesuviana in the Naples area, that you might have to use.

Trenitalia covers almost all trains I have used (stayed in 47 cities in Italy). There is another company if you’re taking the train from Bari over to Matera. It’s at the adjacent building to Bari Centrale.

All you need to know
https://www.seat61.com/train-travel-in-italy.htm

Book tix on the official sites listed above or use their apps

Your previous post provided you with a lot of same info
https://community.ricksteves.com/travel-forum/italy/train-travel-f88a5273-771b-4063-8ec5-3d0436fc56b3