The vast majority of braking done by trains is electrical, ie "regenerative" except lots of trains just burn the generated power in a grid of resistors and dump the heat.

If every mile human beings moved in cars was replaced with two miles taken by train, the world would be a less polluted place.

One train can move many people at once (especially for peak commute times when people would be driving single-occupancy cars just to get to an office). It suffer from similar problems, but it's much more efficient at the same task.

I believe most modern trains use regenerative braking for the majority of their deceleration. At least the hybrid and electric ones.

Not feasible outside the metro areas of only the biggest cities in the US. I can tell you're from one of those cities because you act as if most people have a choice. We don't.

I'm from a small European town. And I did say "where other options are available", so I don't get where your hostility is coming from.