Here is a photo of an Armour & Company meat refrigeration car (a reefer) with a hobo “riding the rods.” The car lacks the grab irons on the right side, so we know it’s before the 1911 Amendment to the Railroad Safety Appliance Act.
People sometimes think the truss rods supported the car body, and in a way, they did help, but car builders of that time said they were installed so the body could be tightened. Railroad cars (rolling stock) “weaves” as it rides, sort of like twisting a dish cloth to get the water out. This was very pronounced on wooden cars, and this loosened up the joints.
It was the side frame built into the body that really supported the car however, like an internal bridge. (The exception were depressed center, or “fish-belly” center sill cars, which were mainly supported by the underframe, but paid a price as the car was much heavier.)
Many reefer cars were wood-sheathed long after wood had been replaced by steel on other rolling stock, including box cars. The builders felt that wood was a far superior insulator than steel.
Wood reefer cars had actual insulation inside, and by the 1950s, they had come to realize that the advantage of steel was it kept the insulation dry, which was much more important to their effectiveness.
Meat reefers were typically shorter than 40 feet, even after 40-f00t freight cars had become the standard. I’ve heard all sorts of explanations as to why, including that this length matched the spacing at the meat packing plants (which had overhead rails for loading and unloading).
That may be true, but I think the real reason was that in the steam-era reefers were cooled by ice in bunkers in their ends. Produce doesn’t need to be kept as cold as meat and the ice was up to the task of cooling the whole interior of a 40-foot car, but meat reefers were probably kept shorter so the natural refrigeration would keep the car at the lower temperatures needed for meat.
There were some 40-foot meat reefers in the later days of steam however. With railroading, so much engineering was seat of the pants, so there always seem to be exceptions. (There were also shipments of meat byproducts which didn’t have to be kept as cold.)
The theory of the loading dock limitation doesn’t explain what happened when mechanical refrigeration was introduced and meat reefers grew longer.
Photos, from above: Armour & Company meat reefer with a hobo “riding the rods.” The lack of the grab irons on the right side shows it’s before the 1911 Amendment to the Railroad Safety Appliance Act; Swift Company reefer cars from ca. 1910s; and an early 1940s reefer car.