"tagging" as we know it dates back to the 1960's-1970's in new york, when NYC made tagging the subways nearly impossible in the 1980's they switched to the other rolling billboards out there (ie trains) kids in other citys saw it picked up the habbit and the rest is history
hobos were known to chalk there names on the sides of cars and trackside buildings along with some info(sutch as where there going or what yards to avoid ect.) i remember seeing one or two examples of this practice on the DPL