...the 100 ton freight car truck first placed into service? And, was it first used by any chance, on covered grain hoppers?
I found this article which might answer some of your questions: http://www.interfacejournal.com/features/03-06/trains_mag/TrainsHALArticle.pdf
I looked in Train Shed Cyclopedia #5, a facsimile reprint of pages from the 1940 Car Builders' Cyclopedia. It showed a lot of 70-ton capy hoppers...open hoppers for coal and 2-bay covered hoppers for cement. But the weight biggies were 75-ton GN ore car, 80 and 96 ton DM&IR ore cars, and a 100-ton Utah Copper Co. ore car. The coverage did not specify truck details. I usually think of covered grain hoppers as coming into use around 1955 more or less.
