PRR FM - Prototype Information
The Pennsylvania Railroad FM class flat cars measured 40' long and 9'3" wide. They were made out of pressed steel, and had wood decking (PRR flat car decking was usually made out of unpainted, untreated oak wood). Initially a vertical brake wheel was found on both ends, but eventually one was removed. They initially came with the 2D-F1 (archbar) trucks, but eventually they were seen with 2D-F2 and 2D-F8 trucks. The car weighed 39,000 lbs, and could carry 100,000 lbs. Prototype photos of the plain car are hard to find. There are, however, quite a few photos of the cars modified to carry LCL containers (brackets were installed on the ends, and the stake pockets removed). (photo of #925187 source:

Note that when I built these flat cars, I did not have the "Pennsylvania Railroad Flat Cars" book written by Elden Gatwood and Al Buchanan, which contains a lot of prototype information about this car. Briefly, they were built from 1902 through 1913. They were the most numerous class of the flat cars found in the PRR's collection (around 3,500). Starting in the late 1920s, some of these cars were converted into the above-mentioned container service. In October 1950, the LCL container service was terminated, and all remaining flat cars were converted back to their original intent.
Prototype Information