Prior to Dec. 11, 2011, (except for trains with the picture of an "eye", you could purchase tickets (from the conductor) on the train--the trains with an "eye" levied a 60SF (or more) fine)--now all trains require that a passenger be in possession of a valid ticket before boarding. One can still upgrade though.
This is now slightly confusing, as you have the "eye" (regional, no ticket sales on board whatsoever) and the "eye" (IC/IR, some ticket sales on board in limited circumstances).
Really not sure why they didn't just make them both the same.
Neil