When one buys a ticket for the train (TER?) from Gare Montparnasse in Paris to Chartres, is it a ticket for a specific train and time, or can it be used for any departure? In other words, if I happen to be close to Gare Montparnasse on a Friday and buy a ticket, can I use it on Saturday? I think this is called an "open ticket"?
And a terminology question: In the US, we say we want a "one-way" or "round-trip" ticket. What are these called in France?
Thanks much!