The American Locomotive Company, often shortened to ALCO, was formed in 1901 through the merger of the Schenectady Locomotive Engine company along with seven smaller locomotive manufacturers of that era. The ALCO company designed, built and sold three major types of locomoties during the 20th century: steam, diesel and diesel-electric. Interestingly, the ALCO company became defunct in 1969 -- the same year the original Lionel company halted production of their own trains.
During the postwar years, Lionel focused on only producing a version of the Alco diesel locomotive. The early production years (1950 - 1954) saw the production of a high quality locomotive which was only offered in an AA combination. Later production (1957 - 1969) included a lower quality and less detailed locomotive, but was available in three different offerings: single A, double AA or AB combination.