Is it possible to go to CT as a day trip via train from Florence?
Yes, but you may not see much. It's about a 2hr 30min - 3hr 20min hour train ride to get to Riomaggiore from Florence. This is the nearest CT station to Florence. The shorter time assumes that you can pick up a Cisaplino or ICplus train at Pisa. It's the same coming back. So you could have 7 hours on the train. In the time available in the CT, you probably could walk around most of the five villages, have lunch and take a late train back. You could probably walk the trail between Riomaggiore and Manarola (about 30 minutes). Assume that you leave at 8:00am and arrive around 11:00am, you have most of the rest of the day. You certainly could see what the CT is all about.