Class AC-1/AC-2/AC-3 (Locobase 9296)

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

Originally placed into service as Mallet Consolidations (MC), several classes of 2-8-8-2 cab-forwards were simpled and superheated, being redesignated Articulated Consolidations (but still called Mallets). The first of the updates resulted in a modest overhaul (see Locobase 9295) that clearly didn't satisfy and a new layout was adopted. The new program was undertaken at the same time later AC classes were being produced by Baldwin. Locobase doesn't know which influenced which, but both major upgrades (modifications and new-builds) included the Type E superheater and its larger surface area.

Compared to the later ACs, however, the MC conversions were smaller engines with less capacious grates and fireboxes. The tubes were longer than those of the later AC class engines, as was the combustion chamber.

17 MC-1 (Locobase 339) & MC-2s, 11 MC-4s (Locobase 3358) , and all 20 MC-6s (Locobase 8724) were rebuilt, including the 4000-4001, which were conventional-cab locomotives and did not operate in the Sierras, and the 4041, which was the first superheated and had different data; see Locobase 9296

Class AC-3a (Locobase 9295)

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

Locobase 9296 describes the general course of simpling and superheating that was applied to the several Mallet Consolidation classes in 1928-1927. These all had Type E superheaters and the entire installation represented a substantial makeover.

One year earlier, however, the Espee had begun the project with the less-ambitious update shown here that used a Type A superheater, shorter tubes, and no combustion chamber. It seems clear that the railroad concluded that this was an insufficiently powerful update and turned to a much different layout.

Class MC-1 (Locobase 329)

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

Conventional Mallet compound of April 1909 (works #33340-33341), predecessors of later "cab-forward" Espee articulateds. As US builders and railroads began adopting superheaters, several variations appeared before the Schmidt smokebox superheater became the preferred installation. Among the early designs was a Baldwin set up that placed a relatively large feedwater heater (1,221 sq ft) between two sections of boiler. Its reheater design was an apparatus that lined the inside of the smokebox. Steam exhausted from the HP cylinders was circulated through the 655 sq ft of the reheater before being directed to the LP cylinders.

The design was modestly successful as a heavy freight pusher, but its conventional layout allowed nearly lethal exhaust to swirl into the cab as the engine traveled through the SP's many tunnels and snowsheds. To combat this oppressive condition, the two were later rebuilt as simple articulateds with the cab-forward design.

Class MC-2/MC-4 (Locobase 3558)

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

These were repeats of the MC-1 (Locobase 329), but were the first of the hundreds of Espee "back-ups", articulated locomotives arranged with their cabs leading.

MC-2s followed the MC-1s in production 7 months later in November 1909 as works numbers were 34019, 34043-34044, 34046-34047, 34063-34067, 34093-34097.

MC-4s (4017-4028) followed in April 1911 (works #36490-36493) and May (36524-36527, 36614-36616, 36634), and

MC-6 (4029-4043) were produced in October 1912 as works #38523-38534 and 38711-38713 in December 1912.

Like the MC-1, this class had a small (655 sq ft) reheater for the LP cylinders, but a larger "pre-heater" (1,221 sq ft) that used the same tube layout as the longer firetubes but heated water with exhaust gas.

Later the MCs (Mallet Consolidations) became AC (Articulated Consolidations) when they were simpled in the 1920s. See Locobase 9626.

Class MC-6 (Locobase 8724)

Data from SP Menke All Time Steam Locomotive diagrams supplied in May 2005 by Allen Stanley from his extensive collection. Works numbers were 38523-38534 in October 1912, 38711-38713 in November

Locobase 329 describes the conventional-cab MC-1s, which introduced the Baldwin feedwater heater as a separate section between two sections of boiler and a Baldwin reheater (of HP exhaust steam). To overcome exhaust-fume poisoning, the builder turned the next group of engines around. As the "cab-forward" or "back-up" MC-2 & MC-4, those articulateds retained both features.

The MC-6 class also had the 1,221-sq ft that used the same tube layout as the longer firetubes but heated water with exhaust gas. The builder discarded Baldwin's reheater for a Schmidt superheater, however, which would prove to be the preferred form of reheating steam.

Later the MCs (Mallet Consolidations) became AC (Articulated Consolidations) when they were simpled in the 1920s.