I believe the C&O had 50-ft staggered side with end-door auto-rack box cars thru 1956. Pontiac was using highway truck-trailers in 1950. C&O first loaded tri-levels at Wixom, MI, in Oct 1960.
The C&O rostered approximately 200 50' auto cars equipped with Evans Auto~Loaders thru 1960. None of the C&O, or inherited PM cars, fitted with loaders had end doors at that time.
Pontiac was shipping autos via trucks by the early 1920's as were most manufacturers, though they still utilized rail service from Pontiac, Michigan, for long haul deliveries (typically more than 400 miles).
Assembly plants located at; Linden, NJ., Doraville, Ga., Kansas City, Mo., Los Angeles, Ca., and Wilmington, Del., were shipping autos by both rail and truck into the 1950's as well.
The deliveries from Pontiac's total production (via rail) averaged around 25% from Pontiac and less than 14% from the assembly plants by the early 1950's. Deliveries from the assembly plants by rail were usually within a 300 to 400 mile range.