Validating your train ticket
Validating your train ticket is important in France. It's not sufficient, simply to buy a ticket. You must validate it as well. You do this by inserting it into a machine (three foot high yellow pillar) on the exit to the platform prior to boarding the train.
This machine prints the day and time of travel on the rear thus preventing it being used a second time. The process takes just seconds but failure to do so could result in a fine. 'Compostez votre billet' is the sign to look for and simply means, "validate your ticket".
It does not mean take your ticket home for recycling! Keep the tickets with you until your journey is over, because railway officials can board the train at any time and ask to see them.