Electro-Motive Division's F7 was the zenith of the "covered wagon," or carbody-style diesel. With 3,849 units built between 1949 and 1953, the F7 was both the most popular carbody diesel and its last hurrah. By 1954 U.S. railroads had virtually stopped ordering F-units in favor of hood units like EMD's GP9 and Alco's RS-3. The side walkways of hood units offered better access for maintenance and better visibility for backup moves, making them truly universal locomotives useful for both mainline runs and road switching.