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.



The 45 locomotives were the last steam locomotives built for the Union Pacific. They represented the epitome of dual-service steam locomotive development; funds and research were being concentrated into the development of diesel-electric locomotives. Designed to burn coal, they were converted to run on fuel oil in 1946. They pulled a variety of passenger trains, such as the Overland Limited, Los Angeles Limited, Portland Rose and Challenger, until diesel-electric locomotives took over passenger service. Many FEF Series locomotives were later reassigned to freight service during the last few years of their careers.

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.