There are plenty of scales in the coal fields, for example. Coal, ore, and other bulk commodities are sold by weight. After a car is loaded it is weighed, with the weight of an empty car subtracted, to find out how much each shipment is and how much has been purchased. There are countless references to railroad scales in any of the numerous books and (presumably) webpages on the subject of coal field railroads.
Seems to me all one would need to model a scale would be a small structure for the operator to be housed in, a concrete pad to simulate the scale, and some simulated by-pass rails to keep the locomotives off the "scale." (Remember that it's not the weight of the locomotives that needs to be rerouted but the torque they exert on the rails that would damage the equipment.)