4-8-8-4 the Western Pacific Railroad, which already rostered large 2-8-8-2s and 4-6-6-4s, but diesel locomotives were gaining popularity and soon were able to displace these Big Boy locomotives. 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
2-8-8-8-8-8-2 2-8-8-8-8-8-2 Under the Whyte notation for the classification of steam locomotives, a 2-8-8-8-8-8-2 has two leading wheels, five sets of eight driving wheels, and two trailing wheels. Because of its length, such a locomotive must be an articulated locomotive. It is not longer than a normal articulated; the fourth and fifth sets of drivers are located under the tender. Other equivalent classifications are: UIC classification: 1-D-D-D-D-D-1 (also known as German classification and Italian classification) AAR classification: 1-D-D-D-D-D-1 French classification: 140+040+040+040+041 Turkish classification: 45+44+44+44+45 Swiss classification: 4/5+4/4+4/4+4/4+4/5 The equivalent UIC classification is to be refined to (1'D)DDD(D1'). The Baldwin Locomotive Works