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.
