PRR X29 - Prototype Information
External Reference:
The Pennsylvania Railroad X29 class is a 40-foot box car. They were built over a period of ten years from 1924 to 1934. They had an 8.5-foot interior height. As initially built, these cars came with Creco doors, KD brakes, and Carmer cut levers (as shown in the photo below). A total of 18,000 of those were built, in five different road number series. Starting in 1928, the cars were equipped with Youngstown doors. A total of 11,476 of those were built. Starting in 1932 the cars were being equipped with AB brakes (source of data), which was completed on almost all cars by the late 1940s to early 1950s. Note that some cars with the Creco doors remained in service until the 1960s.
Prototype Information
External Reference:
The Hagley Digital Archives has several prototype photos as well, including construction photos.

The January 2007 issue of Railroad Model Craftsman, starting on page 90, has an extensive article by Ted Culotta on the X29. Included are prototype photos, and how to model a couple of different versions of this car in HO-scale. The modeling topics include how to model the KD and the AB brake systems (includes several close-up underbody photos of the modeled brake systems). The second installment of the article, in the February 2007 issue, completes the discussion about the X29.

There is also the railfan web site, which provides some information about the various X29 sub-classes.