A scale test car is a type of railroad car

Railroad car

A railroad car or railway vehicle , also known as a bogie in Indian English, is a vehicle on a rail transport system that is used for the carrying of cargo or passengers. Cars can be coupled together into a train and hauled by one or more locomotives...

 in maintenance of way service. Its purpose is to calibrate the weighing scale

Weighing scale

A weighing scale is a measuring instrument for determining the weight or mass of an object. A spring scale measures weight by the distance a spring deflects under its load...

s used to weigh loaded railroad cars. Cars are weighed to ensure they are within the axle load

Axle load

The axle load of a wheeled vehicle is the total weight felt by the roadway for all wheels connected to a given axle. Viewed another way, it is the fraction of total vehicle weight resting on a given axle...

 limits of the railroad, and to determine (by subtracting the car's unloaded, or tare weight

Tare weight

Tare , from the Middle French word tare "wastage in goods, deficiency, imperfection" , from Italian tara, from Arabic tarah, lit. "thing deducted or rejected," from taraha "to reject" weight, sometimes called unladen weight, is the weight of an empty vehicle or container...

 from the total weight

Weight

In science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude , often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus:...

) 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.