Given that the RER lines run inside the city of Paris in addition to outside, I believe that a regular ticket (or one use of a Carnat) will let you ride an RER train. However, something tells me I will not be able to use a regular ticket for the train all the way to Versailles.
How does this work? You use the ticket to go through the turnstile, or does a regular Metro ticket (or Carnat) not work at the RER-C station (such as the one near the Eiffel Tower or near Musee D'Orsay)?
Thanks!!