When I get to Paris and arrive at CDG airport I found I can get free RER line transport to Gare Du Nord with my Eurail pass, however my hotel is just a little farther along the same line. With the 'contremarque' free pass must I get off at Gare Nord or can I stay on the line? Thanks!
The RER is a strange animal. It is most certainly heavy rail and part of it is run by SNCF so your pass works. However large hunks of it are operated by the RATP (the same gang who run the Métro and buses in Paris) where your pass by no means works. The contremarque de passage is not a ticket but the magnetic equivalent of your pass for the journey so you can go through the gates. You need a valid ticket for the RATP bit. You probably know all this. I've always spoken to the attendant who has always sorted it out for me. I use a different pass because of working on the trains but the principle is the same. Bonne chance!
You would need to buy a single t+ ticket (€1.70) to travel beyond Gare du Nord. Unless you have more days on your pass than need, you're better off just buying a regular ticket. A ticket from CDG to anywhere in central Paris costs €9.10, much less than the cost of a pass day.