Not sure if everyone is aware of this (we sure weren't since we had the make sure you validate your ticket in the yellow machines ingrained in our head), but if you have a ticket on a specific train with seat assignments you DON'T have to validate the ticket. It says on the back of the ticket you ONLY validate if you take a different train (like maybe you missed the one you were supposed to take). Had one other Rick Steves person who chatted with us before an IC train say he was going to validate until I showed him the wording on the back of the ticket. Maybe this is just something we missed reading before, but then again maybe not.
I assume you are referencing Italy. Since the penalty for not validating a ticket in Italy is so severe, the standard advice from most us experienced with Italian rail is to validate everything including your mother-in-law. It is a good habit to develop.
When the ticket is train/date specific then it is obvious that you do not need to validate that ticket because is only good for that train, that date, that time. If you miss that train you cannot use that ticket for a later train by validating it -- it is a dead ticket. Within three hours of missing that train you need to exchange that ticket for a new ticket at a ticket booth.
I'm with Frank. I am aware that I do not have to validate tickets with firm departure times and seat reservations printed on it. But I do. I validate all of my tickets in Italy.
I also validate just out of habit... no penalty for doing so and it brings "peace of mind." This link on riding trains has a picture of the "infamous" validation machines: CLICK HERE
As a frequent traveler in Italy...Validate, Validate, Validate. it will only take a second and will avoid any problems. You can find the yellow validation machines at the head of each track and other places in the stazione. They are about the size of a fire alarm. Sometimes I place both ends of the ticket into the machine for the stamp.
Buon viaggio,
RB
Just returned 2 days ago from Italy. We validated ALL of our train tickets even on the Eurostar and InterCity trains with reservations.