Catching the train between Florence and Pisa can be especially useful for people flying into Pisa airport on one of the budget airlines. The cheapest option for those wishing to travel on this route is to take a Regional (Regionale) train between the cities. These services are perfectly comfortable, although seats aren't reserved.
Regionale Trains:
Italian regional train services (Regionale), operated by Trenitalia are the most basic train services in the country. As a result, they are a little scruffy, not that modern, but also surprisingly cheap. There are both first and second class options. Generally, seats are arranged in fours (sets of two facing each other). Reservations are not required, although you obviously must sit in second class if you only have a second class ticket. Carriages are sometimes air conditioned (depending on whether it is working or not!), and are generally safe, although don't be surprised to be visited by beggars on some of the more popular routes.