The first Mallet with the 2-6-6-2 wheel arrangement was the L-1 class of the Great Northern. They were created in 1906 by adding a leading and a trailing axle to the 0-6-6-0 in order to improve running in curves. They were used as helpers in the Cascades. After the first five, 20 more followed in 1908 after the L-2, which had been delivered in the meantime, turned out to be too light. The class Z of the Northern Pacific was identical to these.

Although they had a Belpaire firebox, its heating surface was very small compared to the total heating surface. At least two L-1s received a superheater around 1915, and they also received significantly larger high-pressure cylinders. However, as early as 1925 and 1926, all L-1s were rebuilt to class O-6 Mikados, which remained in use until the mid-1950s.