This reminds of the way the S-Bahn system works through central Berlin. Several different lines all converge on the same route through the heart of the city, providing "adequate" service intervals to their actual endpoints, but as a side effect, 90 sec departures in the city itself.
That's not how the RERs work. The only shared track between any RER lines is the short tunnel for the B and D lines between Chatelet - Les Halles and Gare Du Nord. There have been many plans about doubling it, but it would be extremely expensive because it's under one of the densest and oldest parts of the city, and cross multiple metro lines, and connecting to two of the largest transit hubs in Europe and the world outside of East Asia.
Every other line has dedicated tracks. There can and often are branches, but usually they're 2-3 at most per "side" of the central trunk, with the exception of the unwieldly spaghetti that used to be the RER C between the simplification of it, which reduced it to three branches per side as well.
* RER A in question has two branches to the east, and three to the west.
* B has 2.5 to the south and 2 to the north
* D has 2-3 to the south and one to the north
* E has two to the east, and for now one to the west (I think there will be 1.5 in the end when it's all finished)
So not at all comparable to the S-Bahns that more often than not get all forced into a single tunnel resulting in atrocious frequency on the branches. As an example, frenquency on the branches in rush hour on the A is 5 minutes at most on the least frequented branches. At the worst of times, late at night on the least frequented branches of the least frequented lines you're looking into 20-30 minutes frequency, which isn't great, but isn't terrible either for a station in the middle of nowhere far away on the C, D or E.
> 5 minutes at most on the least frequented branches
I suppose I should say at this point that I live in the USA. And not only in the USA, but in New Mexico :)
> late at night on the least frequented branches of the least frequented lines you're looking into 20-30 minutes frequency
Ditto. This doesn't sound that different from what I remember growing up on the Central Line coming in from the NE of London. Doesn't sound that bad, though more is generally better, I suppose.
It's like a magic trick!