Hard to tell--what size luggage will fit in the overhead on a Trenitalia train?

Would a 24 inch luggage fit or will that have to be placed in the racks in between the cars?

There are numerous train types on Trenitalia trains. All have different luggage storage configurations. I think a 24-inch suitcase would be a problem with most of the overhead racks. I usually travel with a smaller suitcase, and even then I generally use a rack at the end of the car, or else put the suitcase behind the seats.

Would you be able to lift a full 24-inch suitcase over your head, anyway?

I don't think a 24" will fit - but there is usually room at then end of the car.

Depends on width, not length.

A backpack fits overhead, if that helps.

But no big deal, you just put suitcases between the seats.

I have seen 26" on overhead rack on some train. You have to be tall and strong to be able to do this.

There are pyramid shaped spaces between back-back seats you can put luggage also. When I end up in the same car with tour groups all toting 26", I usually end up putting our 20" luggage overhead and give space behind my seat to these souls laden with 26" luggage.

On super crowded trains, I see even larger luggage blocking the corridors. Ticket inspector don't seem to be bothered. They just walk around these behemoth luggage.

Not all trains have seats with "betweens", and those that do have space for one suitcase for every four seats.

We just rode on the Frecciarossa and some regional trains. From my recollection the regional trains have really large overhead bins and we put our quite large suitcases in them. It was quite a lift to do so. Also nice to have your luggage close by when people are getting on an off at multiple stops. With the high speed train the racks weren't quite as big so we used the luggage racks at the end of the car - those got quite crowded.