These cars carried containers for transporting Carbide. Used for making acetylene, & the reason for the big steel containers was because if water came in contact with carbide, it would ignite. I used to rerail a lot of those type cars & containers, & couldn’t use a torch near them. Today, some of those type containers are shipped inside of boxcars, but there are still some containers loaded on flat cars.
So how common were these loads? Lionel made a lot of these gondolas that had these tanks but I hardly ever see them in prototype photos? Perhaps I should move the question to another forum…