Trains have a significant friction advantage over trucks. The degree of “stickiness” between two surfaces is expressed mathematically as the coefficient of friction. For a steel wheel rolling over a steel rail, its value is approximately 0.001. For a rubber tire rolling over pavement, the coefficient is between 0.006 and 0.010, or roughly an order of magnitude greater. Some friction is good — it stops the vehicle when a person runs out in front of it. But too much friction means less energy driving the vehicle forward.
The strength of steel gives trains another efficiency advantage. Rubber tires sag under 80,000 pounds of freight, the amount carried by many trailers. The weight of the truck deforms the pavement, and the road cradles the flattened wheel. This increased surface-area contact means yet more friction. Train wheels and railroad rails deform, too, but the stiffness of steel limits the damage.
As for aerodynamics, trains also trump trucks. Every vehicle has to “punch a hole in the atmosphere,” explains Christopher Barkan, executive director of the rail transportation and engineering center at the University of Illinois at Urbana-Champaign. Once a tractor-trailer has punched its way through, that hole closes. The next truck must punch a new hole. Trains can carry more than 100 trailer-size containers. When the locomotive punches its hole in the atmosphere, each car that follows can sneak into that same hole, saving a tremendous amount of energy. The faster a vehicle travels, the more significant these aerodynamic effects become.