Our party plans to travel by train from Cinque Terri to Siena in May. I remember watching a Rick Steves Travel Tip segment describing him getting on and off a train for an hour or so, then getting back on one toward his original destination. Is one still able to do this? The only Trentalia booking I can find, it seems to discourage this practice.
I think Rick was demonstrating how a rail pass can work for non-reserved trains. For trains with reservations, you may be able to book trains with scheduled stopovers. I did this last year on OEBB/DB traveling Vienna to Frankfurt, booking a 5-hour stop in Wurzburg -- haven't tried on Italian trains. But it was booked in advance, not done on the fly.
Yes, I am sure that was a rail pass not a single ticket but in your case does matter?
Where do you want to get off that is a stop between Siena and CT on route of the train?
You will definitely have a stop and train change in La Spieza.
And I see you'll have luggage: another reason why I wonder why you want to get off for an hour if you'll be hauling it around with you?
Yes, you can if you have a Regionale train ticket from A to D. Regionale tickets are non-reserved seats and you can get on and off in the direction of travel within the valid time of the ticket. I believe Regionale tickets are good for five hours after being validated (time stamped) in the little blue/green/yellow box in the first station. All other trains in Italy require a seat reservation so it is not possible to hop on and off any of the other trains.
However --- big however -- buy your Regionale ticket only at the station DO NOT buy on-line. On-line Regionale tickets come pre-validated with a much shorter time window.