Wheel footprint

Basically the size of the footprint is a function of the weight (force) the wheel applies to the rail and the strength of the wheel and rail materials. The wheel and rail will distort until there is enough contact to support the weight.
As a first cut, if the wheel and rail strengths are 50,000 psi and the wheel load is 25,000 psi, then you will have a contact patch of one half a square inch. The process is actually more complicated as the shapes are not both flat and most likely the strengths are not equal but you should get at least an order of magnitude value this way.
Traditional friction calculations do not use the area of contact as a factor, only the coefficient of friction and the normal (perpendicular) force.