Southern Pacific class MM-2

Infobox Locomotive
name=Southern Pacific class MM-2
powertype=Steam
railroad=Southern Pacific Railroad
gauge=4 ft 8½ in (1435 mm)
railroadclass=MM-2
driversize=63 in (1.600 m) diameter
hpcylindersize=25 in dia × 28 in stroke
lpcylindersize=38 in dia × 28 in stroke
weight=384,800 lb (174,540 kg)
weightondrivers=320,100 lb (145,190 kg)
boilerpressure=200 psi (1.38 MPa)
tractiveeffort=74,070 lbf (329.3 kN)
whytetype=2-6-6-2
feedwaterheater=4B Worthington
roadnumber=4200 – 4211, renum 3900 – 3911
builder=Baldwin Locomotive Works
serialnumber=36684, 36685, 36687-36689, 36703-36705, 36726, 36727, 36740, 36783
builddate=July-August 1911
firstrundate=September 19, 1911
retiredate=1946 – 1948
disposition=scrapped

Southern Pacific Railroad's MM-2 class of steam locomotives was Southern Pacific's (SP) only class of 2-6-6-2 locomotives ordered and built as cab forward locomotives. They 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 making them 4-6-6-2 locomotives. This was done to improve handling at speed. These locomotives were the predecessors of the AC-12 class 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, shops for a couple years before being rebuilt with 4B Worthington feedwater heaters 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.

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.

References

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