CDG to Paris with luggage on the RER. Where do you put your luggage ? I will have one carryon size and a tote.
On your lap or the seat next to you or on the floor at your feet, but never out of sight.
Never put your luggage on the seat next to you. It is rude and should the train be full it takes a seat away. A carry on bag will fit easily on the overhead rack.
One suggestion: If you decide to stand or sit with your suitcase on the floor at your feet, try to keep your hand on the suitcase handle at all times.