Southern Pacific 4-8-8-2 "Cab Forward" Locomotives in the USA

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 Rail Data Exchange. (Thanks to Chris Hohl for his 16 April 2017 email and Baldwin negative attachment revising the loaded tender weight considerably upward.)

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. Firebox heating surface area included 141 sq ft (13.1 sq m) in the combustion chamber.. Piston valves measuring 11" (279 mm) in diameter supplied each of the four cylinders. The design also included a Worthington 6SA feed water heater.

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 Rail Data Exchange. See also DeGolyer, Volume 82, pp. 590+ (AC-4), 622+ (AC-5), 653+ (AC-6) and "Southern Pacific's New Powerful Locomotives," Railway Journal, Volume 35, No 1 (January 1929), pp. 22-26.

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. The RJ report saluting the delivery of the first batch said the general specs were drafted "under the supervision" of General Superintendent of Motive Power George McCormick. The cost of each locomotive was $1.4 million.

Weight grew with each batch. The specs show the lightest-weight AC-4. All had Worthington 4 1/2 BL 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 4 1/2 BL on the fireman's side.

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 Rail Data Exchange. 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 6S.

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 Rail Data Exchange. See also "Southern Pacific Buys New Articulated Locomotives", Baldwin Locomotives, Volume 15, No 3 (January 1937), pp. 7- . Works numbers were 61952-61963, 62038-62051 delivered 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. Some of that weight came from the new one-piece bed castings of both engine units produced by General Steel Castings. Each unit included integral cylinders, their saddles, crossties, valve motion bearers, and the entire keel of the locomotive.

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. These were fitted with the Worthington 6SA feed water heater.

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 Rail Data Exchange. Thanks to Chris Hohl for his 27 February 2018 noting the correct boiler pressure and for supplying tender details and total wheelbases.)

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. The railroad added a Worthington 4 1/2 BL feed water heater. In addition to the tender shown in the specs, the railroad used two others that held less water and slightly less oil.

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.