Actually the first and second rings of Parisian suburbs are certainly populated with a density high enough to justify heavy and dense public transportation ; what is lacking is the political will. Quite a few lines were projected to be long extensions of metro lines or new metro lines ; but they were transformed to tramways, which will be too slow and with too low capacity.

Indeed, the first ring of suburbs is denser now that the outer parts of Paris itself were in the 1900's when the current metro system was built ; the suburban railway lines were built at a time when the suburbs were mostly countryside, and the network is now not dense enough.

And it's also another argument for more progressive tax : another data point on how the wealthy are able to capture and use more state financing than is usually thought.

Un roi sans divertissement est un homme plein de misères