That is a line that I always try to avoid :). I was forced to use it for a short period of time, but I was lucky, I traveled only on the section from Montparnasse to Chatillion which, I think, is not as busy as the rest of the line because most passengers transfer to other lines at montparnasse.
i used to do place de clichy / st ouen at 7pm a long time ago, and i sometimes had to stay on the dock for 20 minutes letting train pass.