After the Seaboard Air Line had already put ten USRA Light Mikados into service as the class Q-1, a total of 117 copies were built as the Q-3 from 1923 onwards. They differed from the USRA locomotives primarily in the firebox and had a longer wheelbase between the leading axle and the first driver. They also had a mechanical stoker and a booster on the trailing axle. After the 117 locomotives had been built by ALCO and Baldwin by 1926, the SAL manufactured number 451 in its own workshops in 1930. This was created from the frame of the 441, which had fallen victim to a boiler explosion. The last Q-3 disappeared in 1955.