It's my belief that some of the earliest uses of covered hoppers and the
concept of "captive service" (other than coal/ore hoppers and other 'purpose
built' cars) was cement service. The earliest covered hoppers that I know
of were the small ones used on the C&O (or was it B&O ... I can never keep
those two roads straight ... *VBG*).
It was a long time later that other products began to be shipped in covered
hoppers and they gained their "ubiquitous" status.
Am I right?