An obvious question: why were the tank cars cleaned? The need is obvious if it carries foods or if the commodity is changed, but in most cases of tank cars used by refineries I would think the car carried the same thing over and over, so why bother cleaning it?
You're right that cars carrying the same thing over and over weren't cleaned unless they required tank repairs of some sort. But many tank cars were operated on short term leases and the commodities they carried changed fairly often. In the case of such loads as motor fuels, lubricating oils, etc. even a little contamination from previous loads was unacceptable.
