We will be making 3 different train trips in May: Paris to Caen, Beaune to Chartres, and Chartres to Paris. Should we buy these tickets here in the U.S. before going, or should we wait to get there and buy them when needed? Do trains fill up at that time of year in France making it more difficult to get tickets at the station just before leaving? Thanks!
Ray, these trains run frequently and I think you can safely buy day of that time of year, as long as your itinerary will not crash and burn should you wind up on a later train. In generally I think May is still a time of year when flexible travelers can travel without reservations or advanced tickets. The chief advantage of buying ahead is that there are some good discounts available 90 days in advance on some routes, if you can commit to nonexchangeable tickets.
You should be aware that there are four holiday weekends in May this year: May 1 Labor Day, May 8 Victory in Europe Day, Ascension May 17, and Pentecost May 27. There will be heavier train travel and the stations will be busier on the weekends surrounding these holidays. Prices will be higher or lower depending on the day and time of day you want to travel due to higher or lower demand. You should look at the calendar and schedules in advance if you want to avoid those high-traffic days. Chartre-Paris is local train, almost within commuting distance, so really not a concern.
Paris-Caen: Buying tickets at the last minute shouldn't be a problem, but you can get a discount fare by booking in advance. Beaune-Chartres: You will have to connect in Paris. By booking in advance you can get a discount fare for the Beaune-Paris leg. The Paris-Chartres leg will be on a regional train. There is open seating and no reservations are possible. Chartres-Paris: Regional train. No advantage to booking in advance. To get discount fares, book at tgv-europe.com. To keep the site in English and to avoid being bumped to the Rail Europe site which doesn't offer discount fares, first choose Great Britain as your country of residence. You will be asked if you want to go to raileurope.co.uk or stay with tgv-europe.com. Choose the latter. Choose Great Britain as your ticket retrieval country. Even though you chose Great Britain, you'll either be able to print your own tickets or pick them up at any SNCF station in France.