And taken from here just for further knowledge: PRR Class X23 boxcars
http://prr.railfan.net/photos/kebaythumbs/X23__40001.jpg
The class X23 box car was adopted by the Pennsylvania Railroad as a standard boxcar in 1912. It was their first 40-foot standard box car (nominal length), and was also their first built with a steel underframe, steel body framing and wood sheathing. Technically it was known as a composite single-sheathed house car. It was also the first PRR box car to be built using a combined alpha-numeric designation as all previous PRR box car classes had been identified using only alphabetical characters (XA through XL). The class X23, as built, had a light weight of about 48,000-pounds, a nominal capacity of 100,000 pounds and a cubic capacity of 2,682-cubic feet. The first car (PRR 40001) was built by the railroad’s Altoona Car Shops and was placed in service during November 1912. Between November 1912, and December 1915, when the last car was completed, 6,911 class X23 box cars had been built by five different organizations. The railroad had built 1,011 cars at its Altoona Car Shops, the Pressed Steel Car Company had built 3,000 cars at its McKees Rocks plant, the Western Steel Car & Foundry Company had built 1,700 at its Hegewisch plant, the American Car & Foundry Company had built 600 at its Berwick plant and the Standard Steel Car Company had built 600 cars at its Butler plant.
The basic configuration of the X23 boxcar was also the basis for the class R7 refrigerator car, of which the PRR had 3,304, the class class K7 stockcar of which the PRR had 25, and the class class X24 automobile car, of which the PRR had 2,000.
In 1943, during World War II, the PRR was experiencing a shortage of cabin cars (cabin car is a PRR term for caboose ) so 75 class X-23 box cars were converted to cabin cars and designated as class NX23.