I think you’re reading that wrong. Trains are nearly the lowest energy/kg per unit distance travelled, as they’re in the bottom of the graph. Though it’s a bit

I think you’re reading that wrong. Trains are nearly the lowest energy/kg per unit distance travelled, as they’re in the bottom of the graph. Though it’s a bit weird for the x axis to be mass, too. Basically the y axis is a measure of efficiency. The x axis just spreads out the points so you can compare things of similar mass.