Train tickets in advance?

Is this for next year as it is already past mid-May? Yes, buy on line if you want to get a discount on major routes and can live with a nonrefundable ticket. 4 months is the farthest out you can buy. Regional trains, just buy as you need as there is no discount.

Your journeys to Siena and the Cinque Terre will involve regionale trains. You can't buy these tickets online in advance unless you are within 7 days of travel. It's so easy to buy when you get to Italy. The regionale trains cannot be sold out. Like a bus in Seattle, you can stand if necessary. The regionale trains don't run full anyway until you get to the CT in summer. It's a zoo there. The trains are quite full but its part of the local atmosphere and it's only a couple of minutes between towns.

Thanks everyone, and oops, I meant Mid-JUNE through mid-July.

You have the exact same itinerary as the one we just came home from :) We only used trains from Venice to Cinque Terre and then to Florence (used the car for Tuscany and to get into Rome). I bought all of my tickets in advance because I'm just that way, but nothing that we were on was sold out. But at the same time, it was nice to just get on the train and not have to stand in the long lines to buy a ticket. Also, you don't know which way the train is going to be facing, so trying to pick a seat that you think is facing a certain way (I'm not a fan of riding backwards) is pointless. The upside is that I'm not sure most people pay attention to seat assignments and it wasn't sold out, so you can always switch.