The vast majority of traffic on the line was, you guessed it; coal! The Chartiers Branch served 25 mines in 1916, as well as received much coal from the other railroad connections. Heavy industries later developed along the line, which included steel plants, glass plants, brick works, lumber mills, chemical companies, mills, a transformer manufacturer, a bridge manufacturer, and various other manufacturing facilities. Coal deliveries were made to power houses along the line, as well.
A great reference for 1920s modeling is this page.