None. Why would you think that? My guess is you're an American living nowhere near an urban rail system but I thought most people here would at least be passing familiar with modern trains. Even some American cities have them.
I've lived and travelled in a ton of places. Trains in low density cities are simply not working well enough. I now prefer to live in exurb and drive everywhere. It's so good.
Pretty much by definition, yes.
Repeating "trains" in every transport context is unproductive. Each mode of transport requires certain density. Most US cities just don't have it. It's that simple.
Which American cities have notable modern train systems? Not Portland, or NYC, or Washington DC.
adjective
no· ta· ble ˈnō-tə-bəl for sense 2 also
1 a : worthy of note : remarkable
| a *notable* improvement