4-8-8-4 4-8-8-4 A 4-8-8-4 in the Whyte notation for the classification of steam locomotives by wheel arrangement, is a locomotive with a four-wheel leading truck, two sets of eight driving wheels, and a four-wheel trailing truck. These are commonly known as "Union Pacific Big Boys". Other equivalent classifications are: The equivalent UIC classification is refined to '(2′D)D2′' for Mallet locomotives. 4-8-8-4 Big Boys were only produced for the Union Pacific Railroad. Twenty-five such engines were built between 1941 and 1944, numbered 4000 to 4024. Eight of these locomotives survive, seven of which are on static public display at various sites in
2-8-8-2 still used on steel mill lines and other railroads until 1983. Other equivalent classifications are: UIC classification: 1DD1 (also known as German classification and Italian classification) French classification: 140+041 Turkish classification: 45+45 Swiss classification: 4/5+4/5 The first 2-8-8-2 was built in 1909 by Baldwin, who sold two to the Southern Pacific Railroad (classified MC-1), and then three each to the Union Pacific Railroad and UP-owned Oregon Railroad and Navigation Company. Baldwin conceived the type as an expansion of the 2-6-6-2 permitting a greater tractive effort. The next order for the type was from the Southern Pacific; these differed in being