I really hate this argument. America use to have tons of rail. People today say "America is too spread out," but that didn't just happen. It grew that way.
Cities like Jena, Germany have ~100,000 people, yet it has 4 tram lines and three intercity stations. Chattanooga, home of the famous Choo-Choo song, has no Amtrak lines at all (where once it had street cars and was a major intercity rail stop).
In America if we started building rail, we'd watch suburbs collapse. Parking lots are huge blocks of lost real estate. Once you have rail and 1/3 of the lots are continually empty, you'll see stores selling that space to developers. Suburbs can being to collapse, simply because of the insane amount of parking lots we have.
I think Seattle will be good example of if this theory holds true as the ST3 expansion will push deep into a lot of suburbs.
I guess in many areas public transport doesn't make much sense to begin with (low density).
In higher density areas, it can impact the decisions people make. A household having 1 car or no car makes it an easier decision to take public transport if it is cheaper than hailing a self driving taxi (or faster during a rush hour or whatever). It makes it easier to bike to work without worrying about the weather. And so on.