Failure to do so may result in a stiff fine-train travel within France is essentially based on the honor system. In most cases a conductor will check your tickets anyway. You can purchase tickets onboard, but you pay a supplement. Tickets in Europe are valid for two months from the date stamped on the ticket. The SNCF also publishes a variety of reduced train fares which they have organized into categories.
Discounts depend on destinations, dates, and times.
Here are the color coded days, cheapest to most expensive. Friday noon through Saturday noon, and Sunday 15h through Monday noon. Under 4 years old is free, but child does not get a seat. Animals pay half fare of a 2nd class ticket or 28 FF if the animal is under 6 kilos and is carried in a bag measuring not more than 45 x 30 x 25cms. If you want to travel with your car or motorcycle on the train, the SNCF has a complete list of destinations applicable to this service.