Train Travel From Rome to Florence

Large luggage is in racks near the carriage entrance. Carry-on size can go in overhead racks above your seat. Also, there is often space for large luggage in the space between back to back seats.
Here is a picture of the interior of a Frecciarosso carriage, note space between seats and overhead rack.
https://www.seat61.com/images/Italy-frecciarossa1000-s-large.jpg
If you are on a Freccia high speed train and you are leaving from Rome Termini, you have one more stop at Rome Tiburtina, then it is nonstop to Florence.

Nobody except tourists worries about big suitcases, wonder why? Because locals know there is no market for used underpants, that train station are camera controlled and that no thief would be so stupid to run away dragging a mammoth suitcase with a screaming tourists in tow and anti-terrorism units guarding all doors.

A thief needs to know who owns what he's going to take before taking it and then he needs to hide it asap. Otherwise anyone near the car's exit door may be the one that starts screaming: "hey that's my son's suitcase, you little..."

The bad guys want your wallet, your smartphone and your tablets; for sure they are not interested in your collection of italian souvenirs and your US clothes.

The last idiot who tried to steal wheeled luggage out of an airport has been denounced by bus drivers, videotaped for 3 days, arrested red-handed and sentenced for repeated theft. Not surprisingly he was an amateur thief, you could almost hear the cops laughing in the videotapes.

In short, stay close to the luggage rack if it makes you feel better. Italians are used to tourists being tourists, but do not block the aisle and go back to your seat when the train leaves. Otherwise other tourists would start wondering what are you doing close to their underpants...

You can stand near your luggage at the luggage rack if you want. When the train departs they don’t make you sit and buckle up like a plane.

Hang around your luggage the entire time if you want, but Dario is correct in saying please don’t block the passage as other travelers also need access.

Well, having born, raised and living in Italy since more then half a century, I can say I know only a single person who had his luggage stolen. It was in 2012, we were in Santiago (Chile), we had put our luggage in the trunk of the bus that had to move our group from our hotel to the airport. When we were at the airport a piece was missing, probably stolen during the wait before leaving the hotel. We are still wondering what a short Chilean thief will make of the shirts of a contrabass player almost 6 feet high.

I lived in Saudi Arabia from 1981-1985 and Germany from 1987-91 and traveled quite a lot those days, as well as since 2010 when I retired.

Back in the 80s we had friends that were on a train in Italy. They had a sleep cabin were they could lock the door. Sometime during the night someone managed to come into their cabin and steal the woman's jewelry case. The only reason that she was carrying jewelry was that the couple was on their way from Saudi Arabia back to the USA as a permanent change of station.

Also, it appeared that they were somehow put to sleep so the thieves could pilfer the jewelry.

I have heard of other thefts on trains, mostly in southern Europe. I would always keep valuables on my person or near me, instead of in a luggage rack out of my sight. Yes, theft of large luggage would be rare, unless someone perceived that you had wealth and very nice luggage. For that reason, we don't spend much $$$$ on luggage.