Can anyone tell me if there is a luggage storage facility in the Chiusi train station? Thanks!
No, there is not. According to the Trenitalia site, the only stations in Tuscany that have left luggage facilities are Firenze S.M.N. and Pisa Centrale.
I was at the Chiusi train station two months ago, and saw none. According to the official Trenitalia website, there are no luggage storage facilities there. (The only two stations in Tuscany with luggage storage are Firenze SMN and Pisa Centrale).
Best option perhaps would be to eat lunch in a restaurant near the train station, and ask the proprietor to hold your luggage whilst you tour the vicinity after (or before) lunch.
Otherwise, it's conceivable (but unlikely) that some bar/cafe might be willing to hold the bags for a fee.