See Locobase 344 for the first several classes of this unique "backup" design. The current entry shows the wartime production batches, which were identical one to another in all but the most minor details. They were the heavyweights of the AC stud, but retained all of the basic dimensions of the prewar engines. One difference was in the amount of superheater area. As noted in Locobase 9294, the Espee later reduced the figure in its postwar diagrams. Locobase suspects the decrease represents removal of some of the elements.
AC-10 -- 4205-4244 (works numbers 64287-64326 in January-June 1942)
AC-11 -- 4245-4274 (64677-64706 in November 1942-May 1943), and
AC-12 -- 4275-4294 (70082-70101 in 1943-1944)
This entry covers the first 25 of 195 built-from-scratch simple-expansion "cab-forward" or "back-up" locomotives supplied by Baldwin to the Espee from 1928 to 1944. Weight grew with each batch. The specs show the lightest-weight AC-4. All had Worthington feed water heaters.
AC-4 -- works numbers were 60575-60576 in August 1928, 60622-60625 in September, 60666-60669 in October 1928.
AC-5 -- works numbers were 60866-60869 in June 1929, 60884-60889 in July, 60952-60957 in August 1929. Weight on the drivers rose to 482,500 lb (max axle loading rose to 61,900 lb) with a total engine weight of 622,600 lb.
The simples succeeded the compound "back-ups" first in service in 1910 (see Locobase 3558). They had 11" (279 mm) piston valves on all four cylinders. The combustion chamber ahead of the firebox contributed 146 sq ft (13.5 sq m) to the direct heating surface area. Feed water heater was a Worthington.
These oil-fired engines had three ostensible disadvantages--and one big advantage--over conventionally laid-out locomotives. The disadvantages were the crew's vulnerability in a collision, the need to keep a high water level in the long boiler to account for grades, and an occasional back blast from the firebox from flameout and sudden relight caused by the long distance from tender to grate. (The fuel was helped through the lines by an 5-lb overpressure in the cylindrical tender.) The great advantage was the reduction of smoke in the cab when travelling through the miles of tunnels and snow sheds between Roseville, Calif and Sparks, Nevada.
This type eventually ranged over much of the Espee system.
Twenty six "backups" had already been delivered according to the specs shown in Locobase 344. With the AC-6s, however, weights rose substantially and boiler construction included thicker-gauge tubes and flues on the last fourteen as boiler pressure rose to 250 psi.. Even so, Locobase finds no obvious differences that accounts for a superheater area reduction of 253 sq ft (23.5 sq m) to 2,735 sq ft.(254.1 sq m).
Like the earlier AC batches, the AC-6s had 11" (279 mm) piston valves on all four cylinders. The combustion chamber ahead of the firebox contributed 141 sq ft (13.1 sq m) to the direct heating surface area. Feed water heater was a Worthington.
As the SP's traffic levels began to recover from the impact of the Great Depression, the railroad went back to Baldwin for more oil-burning "backups". Although the design was essentially the same, it had gained weight over the intervening years. The specs show the AC-7 weight because the AC-8s bulked up to the final weight shown in the AC-10 entry (Locobase 345).
As produced they were credited with the nearly 3,000 sq ft of superheater surface area found in the earlier engines. After World War II, the area dropped to 2,300 sq ft (213.7 sq m) although none of the basic values were changed. This suggests that some of the elements were removed although the flues remained.
AC-8 (62266-62292 in July-October 1939) put 531,000 lb on the drivers and 657,000 lb on all engine axles combined.
Heedless of the debate over their utility recounted in Locobse 11060, the Espee's 4-6-6-2s entered the twenties as compounds. At that point, they were "simpled" and redesignated Articulated Moguls. Obviously satisfactory for certain kinds of service, the locomotives endured through a number change in the late 1930s (to allow grouping of the ACs) and World War II.
Retirement came in 1946-1948 with 4203 being withdrawn in November 1946 and 4207 completing the reduction in September 1948.
|Specifications by Steve Llanso of Sweat House Media|
|Railroad||Southern Pacific (SP)||Southern Pacific (SP)||Southern Pacific (SP)||Southern Pacific (SP)||Southern Pacific (SP)|
|Number in Class||90||26||25||51||12|
|Road Numbers||4205-4294||4100-4125||4126-4150||4151-4204||4200-4211 / 3900-3911|
|Locomotive Length and Weight|
|Driver Wheelbase (ft)||11.33||16.92||16.92||16.92||11|
|Engine Wheelbase (ft)||67.25||66.83||67.25||66.83||54.83|
|Ratio of driving wheelbase to overall engine wheebase||0.17||0.25||0.25||0.25||0.20|
|Overall Wheelbase (engine & tender) (ft)||106.44||106.94|
|Axle Loading (Maximum Weight per Axle) (lbs)||69,100||60,500||69,100||67,000|
|Weight on Drivers (lbs)||531,700||475,200||517,000||514,800||356,900|
|Engine Weight (lbs)||657,900||614,600||639,500||639,700||424,200|
|Tender Loaded Weight (lbs)||320,000||261,000||261,000||295,000|
|Total Engine and Tender Weight (lbs)||977,900||875,600||900,500||934,700||424,200|
|Tender Water Capacity (gals)||22,000||16,152||16,152||16,152|
|Tender Fuel Capacity (oil/coal) (gals/tons)||6100||4912||4912||4912|
|Minimum weight of rail (calculated) (lb/yd)||111||99||108||107||99|
|Geometry Relating to Tractive Effort|
|Driver Diameter (in)||63.50||63.50||63.50||63.50||63|
|Boiler Pressure (psi)||250||235||250||235||215|
|High Pressure Cylinders (dia x stroke) (in)||24" x 32" (4)||24" x 32" (4)||24" x 32" (4)||24" x 32" (4)||22" x 28" (4)|
|Tractive Effort (lbs)||123,364||115,962||123,364||115,962||78,623|
|Factor of Adhesion (Weight on Drivers/Tractive Effort)||4.31||4.10||4.19||4.44||4.54|
|Firebox Area (sq ft)||513||513||513||513||344|
|Grate Area (sq ft)||139||139||139||139||70|
|Evaporative Heating Surface (sq ft)||6505||6505||6470||6505||4367|
|Superheating Surface (sq ft)||2616||2988||2735||2988||1022|
|Combined Heating Surface (sq ft)||9121||9493||9205||9493||5389|
|Evaporative Heating Surface/Cylinder Volume||194.12||194.12||193.07||194.12||177.24|
|Computations Relating to Power Output (More Information)|
|Robert LeMassena's Power Computation||34,750||32,665||34,750||32,665||15,050|
|Same as above plus superheater percentage||44,828||42,791||45,175||42,791||17,910|
|Same as above but substitute firebox area for grate area||165,443||157,927||166,725||157,927||88,012|