Gidday Alex, in my opinion you’re asking a lot of your buildings.[;)][:D]
Glass would have been carried in boxcars.
There is good two part article on the automobile industry and how to model it by Jim Hediger in the May and June 1997 Model Railroader titled *Railroads and the Automobile Age.*It refers to boxcars especially fitted out, and dedicated for parts carriage.
Cheers, the Bear.