Everyone is correct that the big customer on the belt line was Union Carbide. There was also a lumber yard at Johnstone Street and a junk yard. I saw rails in the dirt there once. The Carbide is long gone but the associated power plant is still there as Edison Sault. The beltline was gone when I lived there (1976-84) except for a short section left in at the west end to serve a propane dealer.

The insulated boxcars were always yellow SL-SF (Frisco) cars so I think it was Busch beer from St Louis but I don't know that for sure.