I'm interested in taking a train from Florence to Pisa, visiting Pisa for a few hours, and then continuing to La Spezia. Can I just purchase one train ticket from Florence to La Spezia, get off at Pisa, and then get back on another train to La Spezia later in the day? In other words, can I switch trains with the same ticket or do I need to purchase Florence--> Pisa and Pisa--> La Spezia individually? Will be traveling in late March with a family of 4. Likely using Trenitalia. Thanks.
What is the problem with buying separate tickets? With one Regionale ticket, once you validate (date stamp) the ticket in Florence, the ticket is valid for 4 hours, that means you must complete the journey within 4 hours. If you decide to stay in Pisa for longer, your ticket may not be valid by the time you decide to proceed to La Spezia with a later Regionale train.
If, for the portion from Pisa to La Spezia, you decide to take an InterCity or Frecciabianca, those tickets are train/time specific and cost more, so you can't just hop on one with a regional train ticket. You must have a ticket specific for that train.
Regionale ticket are so cheap that you can buy as you need it. Buy each leg separate. Total cost will be nearly the same as a single ticket. Buy all the tickets in Florence. Then you validate (time stamp) each leg just prior to boarding. Then that ticket is good for 4 hours and any Regional train head in the right direction.
The luggage drop at Pisa Centrale is toward the end of Platform 1, to the right.