The Weyerhaeuser Timber Co. was one of the American logging railroads which used Mallets instead of geared steam locomotives for their steep and winding lines. They hoped that these would be easier to maintain than a Shay, while having the same flexibility. Their first 2-6-6-2ST saddle tank locomotive No. 110 was delivered by Baldwin in 1928 for lines with grades between four and eight percent. It was oil fired for reduced fire risk when running in dry forests. The water tank was split into two parts which were located on sides of the boiler. In accordance with the original Mallet principle, it was a compound.

In the next year numbers 105 and 106 followed, but in contrast to the first one these had four cylinders of the same size. In other respects they were identical to 110, but heavier. In 1929, Baldwin delivered No. 111 that also worked with simple expansion, but had a much larger boiler. Some years later in 1936, Baldwin delivered three more which included the second No. 110, No. 112 and No. 9. These were again nearly identical to the first No. 110, but had larger cylinders.

111 was retired as the first one in 1947. It was sold to Canadian Forest Products and used on Vancouver Island until 1962. The others were used by Weyerhaeuser until the fifties and several were then sold to Rayonier, where they received an additional tender. The first 110 was restored between 1999 and 2001 and is today running on the Black Hill Central. The second 110 was sold to the California Western in 1967 to haul the Super Skunk tourist train. Later it came to the San Diego Railroad Museum.