Southern Pacific 2-8-8-4 "Yellowstone" Locomotives in the USA

Most of Southern Pacifics "big steam" were of the Cab Forward variety (a backwards Yellowstone). However, not all of the SP "big steam" was required to run through the snow sheds of the Sierras. In 1939 the SP received 12 Yellowstones from Lima for use throughout the southern part of the SP system. Unlike the Cab Forwards, the class AC-9 locomotives were built as coal burners. They were later converted to burn oil. Also, the AC-9 class had a grate area of 146 sq. ft. rather than the 139 sq. ft. of their cab forward siblings. With their skyline casings and striped cowcatcher-pilots, they could almost be considered streamlined. They were retired between 1953 and 1956. Sadly, none survived.

Class Details by Steve Llanso of Sweat House Media

Class AC-9 (Locobase 336)

Data from SP Menke All Time Steam Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. Thanks to Chris Hohl for his 9 February 2018 email supplying Lima's Negative 1148 builder's card, which shows higher weights and capacities for the tender.) Works numbers were 7765-7776 in October 1939.

Firebox heating surface included 124 sq ft (11.5 sq m) in seven circulators and several arch tubes.

Conventionally laid-out articulated locomotive for service on the El Paso-Tucumcari section, which didn't require the cab-forward arrangement. Unusually for an Espee locomotive, these were soft-coal burners until they were converted to oil-burning in 1950.

They were essentially "turned-around" cab forwards in which all of the wheelbases remained the same except for the four-wheel truck under the firebox used larger wheels (45" vs 33") that were more closely spaced. At least some of the class were equipped with Hancock size TA2 turbo-feed water heater. The 1951 diagram states that the class were equipped with Worthington 4-B feed water heaters.

Note: Unusually, the Lima table gives a significantly higher adhesion and engine weights than is found in SP locomotive diagrams of a later date. Adhesion weight on the later SP diagram comes in at 522,120 lb, 4 1/2 short tons lighter than the Lima figure and the engine's weight appears as 677,200 lb (307,173 kg) or 12,700 lb (5,761 kg) less. On the other hand, Lima's tender weights are repeated on page 222-R-1 of Menke's Southern Pacific All Time Tender Diagrams collection.

These were fitted with an air-smoothed casing similar to those on the GS-4 4-8-4 passenger locomotives (without the colors). The "raciness" of the profile was not belied by performance--AC-9s were designed to reach 75 mph (119 km/h).

After the Southwest routes were dieselized (in part because of the difficulty in supplying sufficient quantities of good water for steam locomotives), the oil-burning AC-9s operated on the Modoc line in California. Retirements in 1953-1956.

Principal Dimensions by Steve Llanso of Sweat House Media

Locobase ID336
RailroadSouthern Pacific (SP)
Number in Class12
Road Numbers3800-3811
Number Built12
Valve GearWalschaert
Locomotive Length and Weight
Driver Wheelbase (ft / m)33.83 / 10.31
Engine Wheelbase (ft / m)66.25 / 20.19
Ratio of driving wheelbase to overall engine wheelbase 0.51
Overall Wheelbase (engine & tender) (ft / m)112.99 / 34.44
Axle Loading (Maximum Weight per Axle) (lbs / kg)66,400 / 30,119
Weight on Drivers (lbs / kg)531,200 / 240,949
Engine Weight (lbs / kg)689,900 / 312,934
Tender Loaded Weight (lbs / kg)400,700 / 181,755
Total Engine and Tender Weight (lbs / kg)1,090,600 / 494,689
Tender Water Capacity (gals / ML)22,120 / 83.79
Tender Fuel Capacity (oil/coal) (gals/tons / Liters/MT)28 / 26
Minimum weight of rail (calculated) (lb/yd / kg/m)111 / 55.50
Geometry Relating to Tractive Effort
Driver Diameter (in / mm)63.50 / 1613
Boiler Pressure (psi / kPa)250 / 17.20
High Pressure Cylinders (dia x stroke) (in / mm)24" x 32" / 610x813 (4)
Tractive Effort (lbs / kg)123,364 / 55957.04
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.31
Heating Ability
Tubes (number - dia) (in / mm)86 - 2.25" / 57
Flues (number - dia) (in / mm)260 - 3.5" / 89
Flue/Tube length (ft / m)22 / 6.71
Firebox Area (sq ft / m2)589 / 54.72
Grate Area (sq ft / m2)139.50 / 12.96
Evaporative Heating Surface (sq ft / m2)6918 / 642.94
Superheating Surface (sq ft / m2)2831 / 263.10
Combined Heating Surface (sq ft / m2)9749 / 906.04
Evaporative Heating Surface/Cylinder Volume206.45
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation34,875
Same as above plus superheater percentage44,989
Same as above but substitute firebox area for grate area189,953
Power L127,905
Power MT926.50

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Wes Barris