Part of a small array of hand tools displayed in "America On The Move" - such tools were used in the inspection and repair of steam locomotives. Light repairs on steam locomotives were usually done in roundhouses at the many small locomotive terminals throughout a railroad's system; heavy repairs were done in a large, centralized repair shop serving the whole system (often referred to as the "Back Shop"). Most of these tools date from the early- to the mid-20th century, roughly 1900-1955.

Machinists used calipers to measure the sizes and diameters of parts against a standard size or diameter, or to compare a dimension on one part with the same dimension on another, similar part. A large caliper like this one was used by a machinist when operating a large lathe or vertical boring mill, making very large parts. The lathe might be an immense wheel lathe, one that "trued" the wheel treads of locomotive and railroad-car wheels. Or it might be a lathe for axles, in which the outer ends of each axle were machined for later fitting of wheels by a wheel press.