It's a "doodlebug"

Regarding my earlier post about light rail, I learned the proper name for a self-propelled passenger rail car. "Doodlebug."

To make them competitive with buses, though, I still think you'd need to make the rail gauge 7' or so instead of the standard gauge of 4' 8.5". Again, the chief driver of the weight of a rail carriage is the need to keep the center of mass/force between the rails, so a wider gauge allows lighter weight, better stability, higher speed, less vibration, and more.