It's a lot easier to move a loaded filing cabinet across a horizontal floor by using a cart with wheels than to slide it. How much easier? A coefficient of rolling friction μr is defined, which is the horizontal force needed for constant speed on a flat surface divided by the upward normal force exerted by the surface. Transportation engineers call μr the tractive resistance. Typical values of μr are 0.002 to 0.003 for steel wheels on steel rails and 0.01 to 0.02 for rubber tires on concrete. These values show one reason why railroad trains are in general much more fuel-efficient than highway trucks.