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 #64287-64326 in January-June 1942)

AC-11 -- 4245-4274 (64677-64706 in November 1942-May 1943), and

AC-12 -- 4275-4294 (70082-70082-70101 in 1943-1944)

Class AC-4/AC-5/AC-6 (Locobase 344)

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

This entry covers the first 51 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.

AC-4 (works #60575-60576, 60622-60625, 60666-60669 delivered August-October 1928).

AC-5 (60866-60869, 60884-60889, 60952-60957 in June-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.

AC-6 (61353-61354, 61382-61389, 61416, 61426-61429, 61491-61494, 61535-61538, 61543-61544 in May-October 1930). Weight on the drivers rose to 517,000 lb (max axle loading was 69,100 lb) with a total engine weight of 639,500 lb.

. The simples succeeded the compound "back-ups" first in service in 1910.

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-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.

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 although none of the basic values were changed. This suggests that some of the elements were removed although the flues remained.

AC-7 (works #61952-61963, 62038-62051delivered January-August 1937).

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)

Baldwin originally delivered this dozen as Mallet Moguls (MM) for use on passenger trains. Although the wheel arrangement doesn't suggest as much, these were cab-forwards in the same style as the contemporary, and much better known, 4-8-8-2 MCs. When they entered service, they had 23" HP and 38" LP cylinders and boilers pressed to 200 psi.

The single-axle leading truck proved to have insufficient tracking stability and rapid flange wearing in service. One of the class -- 4208 -- derailed while pulling the crack Overland Express. After that accident and several instances of the cab articulating so far that it hid the sides of snowsheds, the Espee fitted a four-axle truck to an extended frame. They retained their class ID of MM-2.

This is where the classification nomenclature fun begins. The Whyte system says that the code should reflect the order of running gear clusters from front to back: A four-wheel truck (4), a six-wheel driver set (6), another 6-wheel driver set (6), and a two-wheel trailing truck (2), or 4-6-6-2.

But others have argued that Whyte's system clearly implied that the front is the smokebox end and the rear is the firebox end. Indeed, photographs clearly suggest a 2-6-6-4 running in reverse. Locobase wonders if 2-6-6-4(R) -- R for reversed -- would have covered things. Of course, there are many other ways layouts have been categorized; Locobase chooses not to engage in that angels-on-pins discussion at this time.

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.