Here is how the efficiency is attained to the best of my knowledge.

The actual contact area of a wheel on the rail (provided they are both contoured properly) the size of a dime.
so that is a contact area of 8 Dimes Per Car.

Friction coefficient of Steel on Steel, Negledgeable

More BS
The optimal contour of the Head of the Rail would be a standard curve.
The Optimal contour of a Wheel would be an abbrievated cone.

The Distance between the Inner surfaces of 2 rails is (Standard Guage) 4 Ft. 8 and 1/2 inches.
The distance between the Flanges is Less thatn (Standard Guage)

Here is the Neat part, When a car goes around a curve it will go to the out side of the outer wheel and the inside of the inner wheel.
now when both of the wheels are contoured as cones on a curve the outside of the wheel diamater will be larger than the Inside of the Inside wheel will be smaller.
Creating in effect a differential. the Outer wheel will travel farther than the inner wheel.

Paul