The Union Pacific FEF Series consists of forty-five 4-8-4 "Northern" steam locomotive types built by the American Locomotive Company (ALCO) between 1937 and 1944 and operated by the Union Pacific Railroad until 1959. Four FEF Series locomotives survive today, including No. 844, which remains in operational condition and now runs in excursion service. Today, the 844 is one of the Union Pacific's oldest serving locomotives and the only steam locomotive never retired by a North American Class I railroad.