Last year I did exactly that. Was in Lucca, wanted to go to Florence for the day. Came to the station, bought a ticket, all very easy, even shows you what train

Last year I did exactly that. Was in Lucca, wanted to go to Florence for the day. Came to the station, bought a ticket, all very easy, even shows you what trains are available and allows you to choose a specific one, which I did. On the train, conductor came past. Must have been a bad day because this lady was furious at us that we didn't stamp our ticket... No idea we had to (since we chose the specific train on the machine...) She was gonna let it slide this time but next time we would pay a huge fine... 7 seconds later we heard her shout exactly the same story to basically every other tourist in the cabin :P Protip to TrenItalia, when printing the ticket, show a big warning on the screen that you need to stamp the ticket to make it valid ^_^.