Under the Whyte notation for the classification of steam locomotives by wheel arrangement, a is a Garratt articulated locomotive. The wheel arrangement is effectively two 4-4-2 locomotives operating back to back, with each power unit having four leading wheels on two axles in a leading bogie, four powered and coupled driving wheels on two axles, and two trailing wheels on one axle in a trailing truck. Since the 4-4-2 type is usually known as an Atlantic, the corresponding Garratt type is often referred to as a Double Atlantic.
The was not a common Garratt wheel arrangement. Only ten were built, all by Beyer, Peacock & Company, the owner of the Garratt patent.
Eight locomotives were built for Argentina to run on .
After nationalization in 1948, all these locomotives were rostered on the General Urquiza Railway.
The first Garratt locomotives to be built to the wheel arrangement were a pair of M class passenger locomotives for the gauge Tasmanian Government Railways in Australia in 1912. They were acquired to haul express passenger trains between Launceston and Hobart.
The two M class engines were the only eight-cylinder Garratt locomotives in the world. They were difficult to maintain and, despite their haulage abilities and speed, both were withdrawn from service some time after the arrival of the R class in 1924 and scrapped in the late 1940s.