Trenitalia unreserved seating?

Unreserved seating is exactly what it says. These trains (regionalle trains) do not have any reserved seating. You just get on, find a seat you fancy and sit down before anybody else sits in it. Just like on a local bus.

"Are you assigned a seat at any time or you just show up and see what is available?" - This sentence sounds like you are expecting some sort of procedure before boarding, like a check-in. Trains aren't like that, you just turn up at the station, stand on the platform and when the train stops get on using the nearest door.

You are not guaranteed a seat, but unless you are travelling at a particularly busy time, there will be plenty of seats. If there are not enough seats, you just stand until one comes free (at the next stop when someone gets off).
It is fairly civilised, but it is Italy.
You can look at Trenitalia and see when the next train is. There will probably be one an hour later, but there will be no need you will get a seat or can stand until the next stop. The tickets for these trains are not for a specific train, hence the four hour window.

Finally, not having reservations is the normal way local trains work everywhere. Reservations are only for the minority of trains that are long-distance.

Don't worry. I don't remember ever having a train in Italy in which I had to stand.

Which of the many train routes to Rome are you taking? The fastest way is to take a regional train to Florence, then change to a Freccia train to Rome, which has reserved seats. a few are all regional trains with a change in Chiusu-Chianciano Terme, or Grosseto. Many, but not all, trains originate at Siena, so show up 15 minutes early and you will be the first on board and have your choice of seats. That will not be the case for your connecting trains, almost all of them will be coming from somewhere else with passengers on board, so you will just have to manage.

So if you want reserved seats, buy tickets with a connection in Florence and you will have seat reservations for the second leg.

Darren,

a few things confuse me.

You said that you made reservations from Venice to Cinque Terre. That have "date, time, train #, car # and seat number."

Only InterCities and above come with reservations, and there are no direct trains to any of the Cinque Terre area (which exact stop are you going to?). You have to make at least one connection, in Milan or Florence and maybe Genoa. Your first train from Venice to Florence (Firenze, Campo di Marte or SMN) or Milan (Milano Centrale) will be a fast train and will have reservations as you say, but what about the connecting train?

Can you be specific please? What do you expect, and what does it say on the ticket? I don't want you to be surprised on a journey half way through when you have to unexpectedly change and then have no reservations for half the journey.

Similar with the trip from Siena. What did you expect and what do the tickets exactly say?

I think you are right Nigel, I'm wondering the same things.

I think the OP did't get the tickets on trenitalia.com/tcom-en by himself, otherwise he'd be able to see how many local trains there are on that route each day.

I suspect an agent who, I hope, sold a ticket from Venezia to the Cinque Terre via Milan with reserved seats all the way to Monterosso. Then, I'm afraid, he must have made some mistakes when booking the second trip from Siena to Rome: he put Darren and Andrea on a local to Chiusi and not on the direct bus to Rome.

It is fairly civilised, but it is Italy.

Jesus man, were you educated in a barn?

Darren, Nigel's post/questions echo my own confusion: This is no "train", meaning a singular one, from Venice to the CT so you'll be changing trains somewhere along the line. Also, the CT is a region encompassing multiple towns so it would help to know exactly which one you're going to and the information on your tickets?

If you're going to be using trains to explore within the CT, those trains are going to be regionales with no reserved seats as well. You'll be purchasing those tickets on a case-by-case basis, and they'll need to be validated before boarding. This is assuming you won't be using a Cinque Terre Treno card. Depending on how busy it is when you visit the region - and it's generally busy all the time during high and shoulder seasons - it IS likely you'll be standing on these trains. It's not an issue, however, as it's just a matter of minutes between towns.

I'm also a bit curious how you're getting TO Siena and from where? Have you also purchased tickets for that piece of your trip?