If I buy at ticket from Paris to Chartres at noon with no reservation and I end up going later the same day on the same route can I use the same ticket. It says its good for the date...so I assume the whole day but it also says non-flexible.
Thanks
yes it is only valid exactly for the one day of travel and the route that you chose when you bought it.
Are you buying the ticket online or are you buying it at a station? If you buy it online, the answer below is correct. If you bought it in the station, the ticket is good for a few days if the train in question is a TER.
Yes, I think you are fine. Many times the top of your ticket says it's good for a period of time starting... In those cases, you won't have any problem catching a later train. Remember to validate the ticket when you board.