Under the Whyte notation for the classification of steam locomotive wheel arrangements, a 2-10-10-2 is a locomotive with two leading wheels, two sets of ten driving wheels, and a pair of trailing wheels.
Other equivalent classifications are:
:UIC classification: 1EE1 (also known as German classification and Swiss classification)
:Italian and French classification: 150+051
:Turkish classification: 56+56
:Swiss classification: 5/6+5/6
The equivalent UIC classification is refined to (1′E)E1′ for Mallet locomotives. All 2-10-10-2 locomotives have been articulated locomotives of the Mallet type.
This wheel arrangement was rare. Only two classes of 2-10-10-2 locomotives have been built: the Atchison, Topeka and Santa Fe Railway's 3000 class, and the Virginian Railway's class AE. The 3000 class performed poorly, so the railroad returned them to their original 2-10-2 configuration after no more than seven years of service. The class AE locomotives were much more successful, providing between 25 and 31 years of service; some were scrapped between 1943 and 1945, and the rest were scrapped between 1947 and 1949. None of either class were preserved.
ATSF 3000 class
In 1911 and 1912, the Atchison, Topeka and Santa Fe Railway modified ten 2-10-2 Baldwin-built locomotives into a new 2-10-10-2 configuration dubbed the 3000 class. They were the largest locomotives in the world from their introduction until 1914. They performed well in helper service, but could only go before losing steam. The ATSF reverted them to their 2-10-2 configurations between 1915 and 1918.