Southern Pacific Company's MM-2 class of steam locomotive

Steam locomotive

A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...

s was Southern Pacific's (SP) only class of 2-6-6-2

2-6-6-2

In Whyte notation, 2-6-6-2 refers to a railroad steam locomotive that has two leading wheels followed by six coupled driving wheels, a second set of six coupled driving wheels, and two trailing wheels...

 locomotives ordered and built as cab forward

Cab forward

The term cab forward refers to various rail and road vehicle designs which place the driver's compartment substantially farther towards the front than is common practice.- Rail locomotives :...

 locomotives. They were built in 1911 by Baldwin Locomotive Works

Baldwin Locomotive Works

The Baldwin Locomotive Works was an American builder of railroad locomotives. It was located in Philadelphia, Pennsylvania, originally, and later in nearby Eddystone, Pennsylvania. Although the company was very successful as a producer of steam locomotives, its transition to the production of...

 and entered service on SP beginning September 19, 1911. By 1914, they had all been upgraded with an additional leading axle making them 4-6-6-2

4-6-6-2

In Whyte notation, a 4-6-6-2 is a steam locomotive with four leading wheels in an unpowered bogie at the front of the locomotive followed by two sets of driving wheels with six wheels each , followed by two unpowered trailing wheels at the rear of the locomotive.Other equivalent classifications...

 locomotives. This was done to improve handling at speed. These locomotives were the predecessors of the AC-12 class

Southern Pacific class AC-12

Southern Pacific Railroad's AC-12 class of cab forward steam locomotives was the last class of steam locomotives ordered by Southern Pacific. They were built by Baldwin Locomotive Works during World War II, with the first, number 4275, entering service on October 27, 1943, and the last, 4294, on...

 cab forward locomotives built during WWII.

SP used these locomotives in the Sierra Nevada Mountains for about 20 years, retiring them in the mid 1930s. They were stored in the railroad's Sacramento, California

Sacramento, California

Sacramento is the capital city of the U.S. state of California and the county seat of Sacramento County. It is located at the confluence of the Sacramento River and the American River in the northern portion of California's expansive Central Valley. With a population of 466,488 at the 2010 census,...

, shops for a couple years before being rebuilt with 4B Worthington feedwater heater

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 of the system...

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

Tractive effort

As used in mechanical engineering, the term tractive force is the pulling or pushing force exerted by a vehicle on another vehicle or object. The term tractive effort is synonymous with tractive force, and is often used in railway engineering to describe the pulling or pushing capability of a...

.

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

Oregon

Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...

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.