We will be traveling from Venice to Siena, spending one night, and then from Siena to Rome. Any suggestions?
Any suggestions for what???? Train vs driving? Places to stay? Not sure what you are asking.
Validate your tickets before you get on the train?
When you get to Siena, walk uphill?
Train
You're still being rather cryptic. For timetables and fares, go to www.trenitalia.com/en/index.html. Although it would be fine to wait until you get to Siena to buy your tickets to Rome, you could buy all tickets in Venice the day before you leave there.