Southern Pacific Company Southern may refer to: Businesses * China Southern Airlines, airline based in Guangzhou, China * Southern Airways, defunct US airline * Southern Air, air cargo transportation company based in Norwalk, Connecticut, US * Southern Airways Express, M ...

's AM-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 loco ...

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

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 ordered and built as

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 actually MM-2s that had been upgraded. MM-2s were built in 1911 by Baldwin Locomotive Works 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 used ...

making them

locomotives. They reclassified their MM-2 as AM-2. This was done to improve handling at speed. These locomotives were the predecessors of 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 primari ...

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 heater A feedwater heater is a power plant component used to pre-heat water delivered to a steam generating boiler. Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency o ...

s and uniform cylinders ("simpling" them) measuring 22 in diameter × 28 in stroke (559 mm by 711 mm). The rebuilds increased the class weight to 424,200 lb (192,410 kg) with 356,900 lb (161,890 kg) on the drivers, 210 psi (1.45 MPa) boiler pressure and 76,800 lbf (342 kN)

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 state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington, while the Snake River delineates much of its eastern boundary with Idah ...

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 AM-2 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