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

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 Raildata collection. 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.

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.

Specifications 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 Wheelbase16.92'
Engine Wheelbase66.25'
Ratio of driving wheelbase to overall engine wheebase 0.26
Overall Wheelbase (engine & tender)
Axle Loading (Maximum Weight per Axle)66400 lbs
Weight on Drivers522200 lbs
Engine Weight677200 lbs
Tender Light Weight320800 lbs
Total Engine and Tender Weight998000 lbs
Tender Water Capacity22100 gals
Tender Fuel Capacity (oil/coal)28 tons
Minimum weight of rail (calculated)109 lb/yard
Geometry Relating to Tractive Effort
Driver Diameter63.50"
Boiler Pressure250 psi
Cylinders (dia x stroke)24" x 32"
Tractive Effort123364 lbs
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.23
Heating Ability
Firebox Area589 sq. ft
Grate Area139.50 sq. ft
Evaporative Heating Surface6918 sq. ft
Superheating Surface2831 sq. ft
Combined Heating Surface9749 sq. ft
Evaporative Heating Surface/Cylinder Volume206.44
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation34875
Same as above plus superheater percentage44989
Same as above but substitute firebox area for grate area189953
Power L127905
Power MT942.47



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