Fantastic feedback - really helpful. Thankyou!!
We are interested in this itinerary as well. Looks like it can be done in 4 hours or so. Is it easy to change trains in Firenze? How much time should we allocate for the connection? We will have a couple of pre-teens and some luggage.
Changing trains is much easier than changing planes. Train stations are much more compact than airports, and there are none of the delays of checking in, security, waiting for luggage or queuing to get out of one little door at front left.
The Trenitalia web site does not show impossible connections. The departure screen will not show the platform for the Venice train until about 20 minutes before departure.
You should plug in the entire trip when you buy the tickets, so Lucca - Venezia (Mestre or Santa Lucia), and let Trenitalia choose the itinerary and connections for you. That way, if the first train is delayed and you miss the second train, they will re-book you for free. If you break up the trip into two legs yourself, you are responsible.