When I lived in Paris, I used (for various jobs) lines 2, 6 and 13 to commute, also RER B. The mere idea of getting a seat during rush hour was delusional.

I remember once, at Denfert-Rochereau, I had to let two RER go to the south without me, as there was litteraly no place for me to go inside. Could not even put a foot in.