^ The entire network of the Paris region is designed and gradually developed to be fit of the population density.
The subway serves the central city and very neighboring suburbs where density is the highest. Oldest lines stop every 200 yards. The lines currently under construction will be natively automated and stops will be slightly more remote from each other. Because people have legs to walk or bike, so they don't need a station every 200 yards.
The RER and Transilien networks serve the Paris region as a whole, far beyond the subway, including outer suburbs with your single-family home neighborhoods, your cars and your American dream.
There's a major difference between RER and Transilien.
Transilien trains get to railway terminals within Central Paris (along with trains serving the entire country) and can't go any further. That's a bad constraint.
While RER trains literally cross the central city and allow you to get pretty much anywhere in the metro area, that is much more flexible and faster in many cases.
Trams are only some lighter and cheaper rail means that try to counterbalance the lacks of the overall network. A mass transit network being a graph supposed to take you from point A to point B as fast as possible.
They are better than buses for more easily recognizable/spottable and their capacity/efficiency is greater, but they are only a cheaper substitute of real heavier rail gear (subway/métro, RER and Transilien).
For instance, the southeastern suburb I live in (Maisons-Alfort) is served by 3 subway stations of line 8, 2 RER stations of line D, soon by automated line 15 of the subway under construction and a whole bunch of bus lines that I hardly ever use. So we are not to be pitied right here because we're neighboring the central city and population is dense.
The problem is suburbs a bit further to the south are not well served yet, so we have many people crossing the town by car to get where they need to go, which is annoying.
I'll show you about something insufficient. They're building this cable car thing from Villeneuve-Saint-Georges to Créteil to provide Val-de-Marne with a bit of more transit means.
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That is really some helpless gadget over here IMO.
Frankly, I don't believe in that kind of solutions as anything effective in Paris. This is not La Paz, Bolivia. There is no mountain at all out here, but there are many more people. We're not facing the same kind of challenges.
They simply found it cheaper to do this cable stuff than to actually solve local infrastructure problems, that would cost billion euros.
Ok, people will say - oh, this is cool. We're flying.
But mass transit is something serious. It's not about entertaining people.
It's about taking them to their everyday whereabouts.
This is not Disneyland, huh.