The larger cars lacked the iron carlines that were added to the 5500 series cars when they were rebuilt. I suspect that the big cars may have had more roof frame issues that contributed to their earlier scrapping... The 5500 cars were all rebuilt a year before the 5900 cars were built, so they effectively the same age. In 1955 when they started scrapping the big cars, they were a bit over 20 years... about the life expectancy of a wood board roof, and a couple of years longer than the 5500 cars waited for their class rebuilding. Add the loading issues that others have mentioned, and the need for the trucks and of course the diminished need for stock cars...

Randy Hees