A scale test car is a type of railroad car in maintenance of way service. Its purpose is to calibrate the weighing scales used to weigh loaded railroad cars. Cars are weighed to ensure they are within the axle load limits of the railroad, and to determine (by subtracting the car's unloaded, or tare weight from the total weight) the amount of cargo loaded. This is used to bill the railroad's customers for the carriage of bulk commodities, so it is essential that the track scales be accurate. Scale test cars are of a precisely known weight so that the track scale can be calibrated against them.
Many scale test cars were small, old railroad cars carrying heavy metal weights as their superstructure. Scale test cars needed special handling so they would not suffer damage, which might alter their weight. They were reweighed periodically on accurate scales at the railroad's shops.