Look for times to Firenze S. M. Novella on www.trenitalia.com - click the British flag first for English, and use a date this week for fares as Regional tickets are not purchasable online more than seven days in advance.
However online tickets are for a specific date and time, then four hours. Tickets bought from any station in Italy, including the ticket machine or booking office at Castiglion Fiorentino, are OK on any train for two months, and for six hours after validation.
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