The Northern Pacific class S-10 (NP S-10)[1] was a type of steam locomotive in use on American railroads in the early 20th century.

The first engines of the type were introduced in 1905, and ten were acquired by the Northern Pacific Railway (NP) in 1907, continuing in service until the 1930s and 1940s.

The remaining fourteen were held by the builder because the railroad entered bankruptcy and sat, mostly complete, awaiting a new buyer.

[2] In 1907, the Northern Pacific was short of power and purchased ten of the fourteen 4-6-0 locomotives remaining at the builder's plant for $14,500 each.

The primary reason was that the light 56-pound rail and the bridge, NP's oldest, over the St. Croix River on the Grantsburg branch could not support heavier locomotives.