Specifically Union Pacific (UP) FEF3 locomotive 842 (larger picture, 172K). FEF3 is an acronym for "Four-Eight-Four, Series 3", the 4-8-4 indicating its wheel arrangement (4 wheels on the lead truck, 8 drivers, and 4 wheels on the trailing truck) and the "Series 3" indicating the third generation of UP's 800 series passenger steam locomotives. Steam locomotives with the 4-8-4 wheel arrangement are referred to as "Northerns" (more information at the ClassicTrains Magazine web site). There are numerous other wheel arrangements as well.
This beauty and her sisters in UP's 800 series pulled crack passenger trains starting in the late 1930's. They were fast and powerful, cruising at 100+ MPH (161+ KPH) while pulling a fully loaded passenger train. Speeds in excess of 120 MPH (193 KPH) were obtained, also fully loaded. Displaced by streamlined diesel locomotives in the mid 50's, they were relegated to freight service where they lived out their lives in fast freight and helper service on UP's most challenging routes. When they were finally retired in the late 50's most fell to the cutting torch, as did the bulk of UP's steam fleet. Fortunately some were saved and made their way into museums or ended up on static display. One still operates today, UP's 844. Built in 1944, it was never taken off of UP's locomotive roster. The 844, and her steam companion, "Challenger" 3985, the world's largest operating steam locomotive, can be found at UP's Cheyenne Wyoming round house. Maintaining and operating these locomotives, as well as UP's other historical equipment, is a credit to Union Pacific, its management, and to UP's Steam Team.