If Tagging is the Language of the Oppressed...

I was studying the box cars on a passing train yesterday and thinking about when my grandfather used to "ride the rails" looking for work during the Great Depression, when this thought occurred to me: If graffiti is the language of the oppressed, then spray paint must be a writing utensil. What utensil did folks use before spray paint? And was there graffiti on train cars back during the Depression? If there was, what would they have looked like?

Things my random brain ponders.