Southern Pacific Company's MM-2 class of

steam locomotive A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, oil or, rarely, wood) to heat water in the locomot ...

s was Southern Pacific's (SP) only class of

2-6-6-2 Under the Whyte notation for the classification of steam locomotives by wheel arrangement, a is a locomotive with one pair of unpowered leading wheels, followed by two sets of three pairs of powered driving wheels and one pair of trailing wheels. ...

locomotives ordered and built as oil-fired

cab forward The term cab forward refers to various rail and road vehicle designs that place the driver's compartment substantially farther towards the front than is common practice. Rail locomotives In steam locomotive design, a cab forward design will ...

locomotives. They were built in 1911 as compound-expansion

Mallet A mallet is a tool used for imparting force on another object, often made of rubber or sometimes wood, that is smaller than a maul or beetle, and usually has a relatively large head. The term is descriptive of the overall size and proport ...

locomotives by

Baldwin Locomotive Works The Baldwin Locomotive Works (BLW) was an American manufacturer of railroad locomotives from 1825 to 1951. Originally located in Philadelphia, it moved to nearby Eddystone, Pennsylvania, in the early 20th century. The company was for decades t ...

and entered service on SP beginning September 19, 1911. By 1914, they had all been upgraded with an additional

leading axle The leading wheel or leading axle or pilot wheel of a steam locomotive is an unpowered wheel or axle located in front of the driving wheels. The axle or axles of the leading wheels are normally located on a leading truck. Leading wheels are us ...

making them

4-6-6-2 In Whyte notation, a 4-6-6-2 is a steam locomotive with four leading wheels (two axles) in an unpowered bogie at the front of the locomotive followed by two sets of driving wheels with six wheels each (three axles each), followed by two unpowered ...

locomotives, reclassified from MM-2 to AM-2. This was done to improve handling at speed. These locomotives were the predecessors of several other cab-forward engines, culminating in the AC-12 class cab forward locomotives built during World War II. SP used these locomotives in the

Sierra Nevada The Sierra Nevada () is a mountain range in the Western United States, between the Central Valley of California and the Great Basin. The vast majority of the range lies in the state of California, although the Carson Range spur lies primarily ...

for about 20 years, retiring them in the mid-1930s. They were stored in the railroad's

Sacramento, California ) , image_map = Sacramento County California Incorporated and Unincorporated areas Sacramento Highlighted.svg , mapsize = 250x200px , map_caption = Location within Sacramento C ...

, shops for a couple years before being rebuilt with 4B Worthington feedwater heaters and uniform cylinders (making them simple-expansion them) measuring ,

diameter In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid for ...

stroke A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop functionin ...

. The rebuilds increased the class weight to with on the drivers, boiler pressure and

tractive effort As used in mechanical engineering, the term tractive force can either refer to the total traction a vehicle exerts on a surface, or the amount of the total traction that is parallel to the direction of motion. In railway engineering, the term tr ...

. The rebuilt locomotives were renumbered into the 3900 series then used on SP's Portland Division in

Oregon Oregon () is a U.S. state, state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington (state), Washington, while the Snake River delineates much of it ...

until they were again retired in the late 1940s. The locomotives were all scrapped soon after retirement with the last, 3907 (originally 4207), on September 23, 1948.

References

* {{SP cab forward locomotives MM-2 2-6-6-2 locomotives 4-6-6-2 locomotives Baldwin locomotives Mallet locomotives Steam locomotives of the United States Railway locomotives introduced in 1911 Scrapped locomotives Standard gauge locomotives of the United States