Unfortunately, the High-Speed Rail has become exorbitantly expensive, all publicly funded a.f.a.i.k. On the other hand, housing in general and the existing 19th-century railroad network were built with private money. As an aside, this is why the internet backbones from coast-to-coast are privately owned; these are on railroad land.
Even a rail line such as RER A only has a capacity of at most 40k people/h/direction. It cannot solve all, or even most, transportation problems of a large city. For that, you'd need tens of lines, as in China or Madrid.
Unfortunately, the High-Speed Rail has become exorbitantly expensive, all publicly funded a.f.a.i.k. On the other hand, housing in general and the existing 19th-century railroad network were built with private money. As an aside, this is why the internet backbones from coast-to-coast are privately owned; these are on railroad land.