Southern Pacific class MM-2

Southern Pacific Company's MM-2 class of steam locomotives was Southern Pacific's (SP) only class of 2-6-6-2 locomotives ordered and built as oil-fired cab forward locomotives. They were built in 1911 as compound-expansion Mallet locomotives 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 making them 4-6-6-2 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.

Southern Pacific class MM-2
Type and origin
Power typeSteam
BuilderBaldwin Locomotive Works
Serial number36684, 36685, 36687–36689, 36703–36705, 36726, 36727, 36740, 36783
Build dateJuly–August 1911
  Whyte2-6-6-2 Mallet
  UIC(1′C)C1′ n4v
Gauge4 ft 8 12 in (1,435 mm) standard gauge
Driver dia.63 in (1,600 mm)
Adhesive weight320,100 lb (145,200 kg; 145.2 t)
Loco weight384,800 lb (174,500 kg; 174.5 t)
Total weight568,000 lb (258,000 kg; 258 t)
Fuel typeFuel oil
Fuel capacity3,200 US gal (12,000 l; 2,700 imp gal)
Water cap10,000 US gal (38,000 l; 8,300 imp gal)
  Firegrate area
70 sq ft (6.5 m2)
Boiler pressure200 psi (1.38 MPa)
Feedwater heater4B Worthington
Heating surface5,292 sq ft (491.6 m2)
CylindersFour: two high pressure (rear), two low pressure (front)
High-pressure cylinder25 in × 28 in (635 mm × 711 mm)
Low-pressure cylinder38 in × 28 in (965 mm × 711 mm)
Valve gearWalschaerts
Performance figures
Tractive effort74,070 lbf (329.48 kN)
OperatorsSouthern Pacific Company
Numbers4200 4211 (MM-2), renumbered 3900 3911 (AM-2)
First runSeptember 19, 1911
Retired1946 1948
DispositionAll scrapped

SP used these locomotives in the Sierra Nevada for about 20 years, retiring them in the mid-1930s. They were stored in the railroad's Sacramento, California, shops for a couple years before being rebuilt with 4B Worthington feedwater heaters and uniform cylinders (making them simple-expansion them) measuring 22 in × 28 in (559 mm × 711 mm), diameter × stroke. The rebuilds increased the class weight to 424,200 lb (192,400 kg) with 356,900 lb (161,900 kg) on the drivers, 210 psi (1.4 MPa) boiler pressure and 76,800 lbf (342 kN) tractive effort.

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


  • Diebert, Timothy S. & Strapac, Joseph A. (1987). Southern Pacific Company Steam Locomotive Conpendium. Shade Tree Books. ISBN 0-930742-12-5.
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