Southern Pacific 4-8-8-2 Cab Forward Locomotives

Class Details by Steve Llanso

Class AC-10/11/12 (Locobase 345)

Data from SP Menke All Time Steam Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive collection.

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)

Class AC-4/AC-5 (Locobase 344)

Data from SP Menke All Time Steam Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive collection. See also DeGolyer, Vol 82, pp. 590+ (AC-4), 622+ (AC-5), 653+ (AC-6).

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.

See AC-6 (Locobase AC-7 (Locobase 9294) and AC-10 (Locobase 344) for later engines in this arrangement.

Class AC-6 (Locobase 14354)

Data from SP Menke All Time Steam Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive collection. See also DeGolyer, Vol 82, pp. 653+. works numbers were 61353-61354, 61382-61389 in May 1930; 61416 in June, 61426-61429 in July, 61491-61494 in September, 61535-61538, 61543-61544 in October

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.

See AC-7 (Locobase 9294) and AC-10 (Locobase 344) for later engines in this arrangement.

Class AC-7/AC-8 (Locobase 9294)

Data from SP Menke All Time Steam Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive collection. Works numbers were 61952-61963, 62038-62051delivered January-August 1937

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.

See AC-4 (Locobase 344) and AC-10 (Locobase 344) for the other engines in this arrangement.

Class AM-2 (Locobase 6688)

Data from SP Menke All Time Steam Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive collection.

Heedless of the debate, 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.


Specifications by Steve Llanso
ClassAC-10/11/12AC-4/AC-5AC-6AC-7/AC-8AM-2
Locobase ID345 344 14354 9294 6688
RailroadSouthern Pacific (SP)Southern Pacific (SP)Southern Pacific (SP)Southern Pacific (SP)Southern Pacific (SP)
CountryUSAUSAUSAUSAUSA
Whyte4-8-8-24-8-8-24-8-8-24-8-8-24-6-6-2
Road Numbers4205-42944100-41254126-41504151-42044200-4211 / 3900-3911
GaugeStdStdStdStdStd
BuilderBaldwinBaldwinBaldwinBaldwinBaldwin
Year19421928193019371911
Valve GearWalschaertWalschaertWalschaertWalschaertWalschaert
Locomotive Length and Weight
Driver Wheelbase11.33'16.92'16.92'16.92'11'
Engine Wheelbase67.25'66.83'67.25'66.83'54.83'
Ratio of driving wheelbase to overall engine wheebase 0.17 0.25 0.25 0.25 0.20
Overall Wheelbase (engine & tender)106.44'106.94'
Axle Loading (Maximum Weight per Axle)69100 lbs60500 lbs69100 lbs67000 lbs
Weight on Drivers531700 lbs475200 lbs517000 lbs514800 lbs356900 lbs
Engine Weight657900 lbs614600 lbs639500 lbs639700 lbs424200 lbs
Tender Light Weight320000 lbs261000 lbs261000 lbs295000 lbs
Total Engine and Tender Weight977900 lbs875600 lbs900500 lbs934700 lbs424200 lbs
Tender Water Capacity22000 gals16152 gals16152 gals16152 gals
Tender Fuel Capacity (oil/coal)6100 gals4912 gals4912 gals4912 gals gals
Minimum weight of rail (calculated) on which locomotive could run111 lb/yard99 lb/yard108 lb/yard107 lb/yard99 lb/yard
Geometry Relating to Tractive Effort
Driver Diameter63.50"63.50"63.50"63.50"63"
Boiler Pressure250 psi235 psi250 psi235 psi215 psi
Cylinders (dia x stroke)24" x 32" (4)24" x 32" (4)24" x 32" (4)24" x 32" (4)22" x 28" (4)
Tractive Effort123364 lbs115962 lbs123364 lbs115962 lbs78623 lbs
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.31 4.10 4.19 4.44 4.54
Heating Ability
Firebox Area513 sq. ft513 sq. ft513 sq. ft513 sq. ft344 sq. ft
Grate Area139 sq. ft139 sq. ft139 sq. ft139 sq. ft70 sq. ft
Evaporative Heating Surface6505 sq. ft6505 sq. ft6470 sq. ft6505 sq. ft4367 sq. ft
Superheating Surface2616 sq. ft2988 sq. ft2735 sq. ft2988 sq. ft1022 sq. ft
Combined Heating Surface9121 sq. ft9493 sq. ft9205 sq. ft9493 sq. ft5389 sq. ft
Evaporative Heating Surface/Cylinder Volume194.12194.12193.07194.12177.24
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation3475032665347503266515050
Same as above plus superheater percentage4482842791451754279117910
Same as above but substitute firebox area for grate area16544315792716672515792788012
Power L12575126690265692669014005
Power MT854.18990.59906.38914.39519.06

Photos

Reference

Credits

Introduction and roster provided by Richard Duley. Class details and specifications provided by Steve Llanso of Sweat House Media.