Venice Train Station

Santa Lucia is in Venice proper on the canal

Venezia S. Lucia is the main train station. Mestre is on the mainland. A few fast trains from other cities (Milan?) only go as far as Mestre and you would have to switch to a "local" ("regionale") for the last 10 minutes to Venezia S. Lucia, but most will take you all the way to Venezia S. Lucia. Use trenitalia.com to see schedules. When you walk outside of Venezia S. Lucia, you'll see that you are on the Grand Canal, and can then take a vaporetto (a water bus; the main form of public transportation) to get closer to your hotel, if you book a hotel in Venice (not in Mestre!).

I have a question - we plan to take the train from Roma Termini station to Venice Santa Lucia station. Where can we book the tickets for the roundtrip journey and also will the tickets need to be validated

mathewmk, these are the two rail services you can use for your journey:

Trenitalia (Italy's national rail service)
https://www.trenitalia.com/en.html

Italo: a private rail company serving mostly the bigger Italian cities:
https://www.italotreno.it/en

Either one will do. You will be (or should be) purchasing tickets for specific long-distance trains, at specific times, with assigned carriages and assigned seats. Any ticket with assigned seats does not need validating.

A good primer on Italian trains:
https://www.seat61.com/Italy-trains.htm

Mathew we used Trentitalia train from Rome to Naples and booked the Frecciarossa train line. We booked round trip and printed the boarding passes right when you book. No need to check in or anything at the train. It was very easy. I'm hoping to book the same train to Venice.