"The floor tubes, that held the chains, for the auto loaders (only), looked like a big pipe with a base plate on it, that stuck down through the floor of the car about 8-12 inches (they vary). They are very visible on cars that had Evans loaders and the like."
It seems to be a common misconception that the floor tubes held just a chain. In actuality, the floor tube was used to store the patented Evans tie down assembly which was a spring loaded, turnbuckle device that was nearly 16" in length. The spring assembly was used on the floor as well as the loader and allowed vehicles to be held in a semi-rigid position. Attached to the main floor assemblies was a 5'-8 1/4" length of chain (as per Evans' standards) used to secure motor vehicles within auto cars. Depending on the type of loader and the length of the auto car extensions were often added to the chain allowing greater flexibility while positioning and securing vehicles to the floor of auto car.
In order to accommodate the tie down assembly, the minimum length of any floor tube was 19 inches and the maximum length was 28 inches. All round tubes were 5 inches in diameter and oval tubes were 5 by 6 inches. The hinged cap assembly on original tube assemblies was a cast brass fitting which (thru time) failed under stress and all subsequent designs (Type "B" thru "F", circa 1934-39) were replaced with forged metal caps.
During the annual modifications or complete replacements of loaders, done in order to accommodate the newest model year offerings from manufacturers, the original tube assemblies were altered to employ the new forged components. If an earlier model of loader was replaced, it was also common to relocate some, or all, of the floor tubes. Much of this work was contracted to Evans' own shops or in railroad shops which purchased the parts from Evans.
I am not sure what is meant by, "And the like." Only two designs of auto loaders were used by the railroads; the Evans Auto~Loader and the two designs of NYC Loaders. The early New York Central design used a tie down system which resembled snow tire chains to secure vehicles on both the floor and the rack assembly (loader). These were unreliable and chaffed vehicle tires, thus workers were forced to wrap tires with hemp or cloth before securing the vehicles. This was laborious and a return to pre loader methods which all involved were trying to avoid. By the late 1930's nearly all early NYC had been converted to Evans tie down assemblies in the same fashion as the second design which incorporated the Evans system (as installed).
Though always listed on the cover page within Bulletin 28, the AAR design of loader was never put into production. One car was fitted for testing, but the design did not win favor with the railroads or manufacturers and the AAR's project was shelved in 1949.