The 4000-series cars were manufactured by the Cincinnati Car Company of Cincinnati, Ohio, between 1914 and 1924. They were the first steel cars on the Chicago "L" system. These cars were built in 2 distinct variants, with the earlier, metal-roofed cars being known as "Baldies" (due to their smooth iron roofs) and the later, wooden-roofed cars being known as "Plushies" (due to their more comfortable, green plush seats). The "Baldies" were equipped with 6 doors per car, however the center doors were never used regularly. The "Plushies" were equipped with only the end vestibule doors, but the cars internal structure was arranged to allow for the later addition of the center doors.