What makes you think blocking the car doors open was for transport of livestock? Do the rules specifically state anything about livestock? While we can document specific or unusual shipment of livestock in boxcars (I too have the photo's Ray mentioned), I suspect this rules were to accommodate those loads that typically were shipped in ventilated boxcars, ie watermelons. The IC served portions of the south. Many southern railroads had ventilated boxcars. These rules may have been a way to compete when a ventilated boxcar was requested and the IC could not provide it.
