We will be travelling from CDG to Luxembourg Station on RER. we will each have a large suitcase. Where can we keep the suitcase of the train? I saw a post indicating that we would have to keep it in the bicycle car and stand up for the entire journey. We are elderly people and cannot stand for the entire time. What to do?
The RER B train that travels from CDG to Paris will most often now have train cars with baggage racks and you definitely do not have to travel on the end cars where bicycles are requested to board.
Since you are boarding the train at CDG which is the origin of the RER B line, you will have your choice of seats and can keep your luggage with you. The RER B train at CDG will be at station for 10-15 minutes, with doors open, prior to departure so you will have plenty of time to board and find a suitable spot to sit with your luggage.
The configuration of seats in the middle of the cars will be four seats facing each other. I think you would be able to fit two large suitcases near the window seats (keeping them on their wheels on the ground, not on the seats) and you and your travel companion can sit on the aisle seats.
There may also be a pair of seats near the doors where two people can sit and keep their luggage with them. Alighting would be easier from the door seats. But, if you're sitting at the mid-car 4 seats, you can begin to prepare for leaving the train when the train arrives at Gare du Nord. When people get up to alight the train at Gare du Nord, move from the mid-car seats to the door seats / area, which will make alighting at Luxembourg (3rd stop after Gare du Nord) easier.
I'd recommend taking an "express" RER B train towards Paris, which doesn't make stops between CDG and central Paris. Therefore less passengers will board these express trains until Gare du Nord, where it will begin to make all the stops in central Paris, including Luxembourg. More info on how to recognize express trains here on the RER B schedules page.
Have a great trip.