Galveston, Harrisburg & San Antonio / Southern Pacific 2-6-6-2 "Mallet Mogul" Locomotives in the USA

No. 950

Just Another Step of Progress Along the Lines of the Sunset Route

This locomotive is the largest and only one of its kind in the south, the largest in the state of Texas, and one of the largest engines in the world. Weight on drivers is 266,000 lbs. Total weight loaded is 469,000 lbs. Tractive power 70,000 lbs. Length is 90 feet. The locomotive consists of two engines placed under one boiler with two sets of cylinders on each side. Twelve driving wheels, and a two-wheeled radial truck at each end, 16 wheels in all. The wheel base is necessarily long. To provide the required flexibility, the front frames are hinged to the rear frames at a point on a center line of the engine between the high-pressure cylinders. The boiler is held in rigid alignment with the rear frames and the forward overhang is supported on the front frams by sliding bearings, hence when rounding a curve the front group of drivers follow the curve while the boiler is held in rigid alignment on the rear frames.

The high-pressure cylinders drive the rear group of wheels, receiving steam directly from the boiler. The low-pressure cylinders drive the forward group obtaining reheated steam supplied from exhaust of the rear cylinders. This locomotive is equiped with two of the largest Westinghouse air pumps ever built, capable of storing 120 cubic feet of air per minute. The cylinder cocks, reverse lever and sanders are operated by air, which permits this engine, although gigantic in size, to be operated with less exertion that an ordinary engine.

Walshaert valve gear is used. The two engines can be worked together or independent of each other, and are capable of handling 3,600 M's up a 2% grade. Water capacity of tender is 9,000 gallons. Oil capacity is 3150 gallones. The use of compound cylinders with reheater results in considerable fuel economy.

Built by the Baldwin Locomotive Works of Philadelphia, PA, this great mass of steel and iron is a tribute to the ingenuity of man. The secondary marvel is that this great factory turns out a complete locomotive, as shown in this picture (see below), built from the ground up, and adjusted like a watch, every 24 hours.

Houston, TX Fefruary 28, 1910

Class Details by Steve Llanso of Sweat House Media

Class MM-1 (Locobase 8728)

Data from SP 7 - 1951 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. See also DeGolyer, Volume 33, pp. 319+. See also "Baldwin Mallet for the Galveston, Harrisburg & San Antonio", Railway and Locomotive Engineering, Volume 23, No. 4 (April 1910), p. 138. Thanks to Tom Dietrich for his 9 August 2016 email sharing a photograph and accompanying 1910 release with Locobase;and to Chris Hohl for his 11 August 2017 email noting the superheater area.) Works numbers were 34235-34237 in January 1910; 34247-34250, 34331, 34378-34381 in February 1910.

The only articulateds to grace the SP system's Texas rails were the dozen oil-burning Mallet Moguls that went into service in 1910 on the GH&SA. Like the MC-1 series (Locobase 329), these were conventional-cab locomotives with a Baldwin superheater in the smokebox to reheat the low-pressure steam (i.e., to restore some vigor to the steam exhausted from the high-pressure cylinders on its way to the LP set.). Omitted from this Mallet, said the R&LE, were the "separable joint, feed-water heater and internal combustion chamber" that many Baldwin articulateds had at the time.

All four piston valves were identical and measured 13" (330 mm) in diameter.

Tom Dietrich's grandfather may have posed with the first of the class (950) in the photograph whose provenance Dietrich outlines as follows: "I can tell you about the history of the photo for at least the past 50 years. I first remember seeing it when our family moved to Fresno CA in 1966 and my dad hung the photo in his workshop, Im sure he had it before that, but I don't remember seeing it prior to 1966. I took possession of it when my dad passed away in 1990, and I have had it ever since. The photo itself is 8.25" x 6" and is framed. The fact sheet is behind glass on the back side of the frame which gives its history."

Headlined "Mogul Mallet Locomotive - No. 950 - Just Another Step of Progress Along the Lines of the Sunset Route" and datelined "Houston, Texas - February 28, 1910", the release celebrates the "largest and only of its kind in the South, the largest in the State of Texas, and one of the largest engines in the World." A big deal for Texas, perhaps, but, according to the R&LE report, these were of "comparatively moderate size for an engine of this type." Its data match those of the 1907 Great Northern L-2s shown in Locobase 3554.

The February 28th release provided a snapshot of the builder, too: "[T]his great mass of steel and iron is a tribute to the ingenuity of man. The secondary marvel is that this great factory turns out a complete locomotive as shown in this picture, built from the ground up, and adjusted like a watch -- one in every twenty-four hours."

The MM-1s were not particularly successful, although it was almost two decades before the railroad dismantled them and adapted their boilers to an 0-8-0 switcher layout.

Class MM-2 (Locobase 11060)

Data from "Oil-Burning Locomotives", Record of Recent Construction #57 (Baldwin Locomotive Works, 1911), p. 7. See also DeGolyer, Volume 37, pp. 200+. "Oil-Burning Passenger Locomotives 2-6-6-2 Type", American Engineer and Railroad Journal, Volume 85, No. 10 (October 1911), pp. 406-407. Works numbers were 36684-36689, 36703-36705, 36726-36727, 36740 in July 1911; 36783 in August.

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, 2-8-8-2 MCs. In addition to the heating surface shown in the specs, this class had the big feedwater heater that occupied 1,590 sq ft (147.7 sq m) in the front section of the boiler barrel. It had 424 2 /14" tubes in the bundle, each 6 feet 3 in (1.905 m) long and a central tube with a 15" (381 mm) diameter.

AERJ describes the steam circuit and how it differed from more conventional Mallet arrangements: "[T]he dome is placed a short distance ahead of the firebox, and an internal dry pipe conveys the steam to the intermediate combustion chamber. This chamber contains right and left hand steam pipes of ordinary construction, and these communicate with short outside horizontal pipes, which lead to the top of the high pressure steam chests. The high pressure exhaust is conveyed to the smoke-box through a horizontal pipe located in a large flue which traverses the water heater. The flexible receiver pipe is placed on an angle under the smoke-box."

Each of the four cylinders was fed steam (fresh or reheated) through 15" (381 mm) inside-admission piston valves. This is in line with one announced goal: "Special attention has been given to the steam distribution, and to providing ample sectional areas in the steam and exhaust piping."

The account concludes with the usual three cheers: "Although the duty which these locomotives are intended to perform is exceptionally severe, there is every reason to anticipate that they will prove successful."

In the event, success proved more elusive than had been hoped. The single-axle leading truck proved to have insufficient tracking stability and rapid flange wearing in service. Baldwin specifications page 88 1/2 summarizes "Mr Buck's" letter outlining the details that he wished to change in future orders. Buck noted that the specification had called for a 48,000 lb (21,772 kg) axle loading, but the engines weighed in at 51,000 lb. In the future, the SP wanted one more plate in each assembly of driving springs. Also, they suggested moving the equalizing beam fulcrums back far enough to create a longer bearing surface. Given that the entire space under the combustion chamber was taken up by the back waist bearer and a cleaning hopper would be difficult to arrange in that space, the SP suggested a "suitable regulation of the draft" might allow the engine to clean itself "thoroughly."

Other comments and complaints receive a detailed response on that same page. It's quite a meticulous review and reply.

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 of the MM-2 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 declines to engage in that angels-on-pinheads discussion at this time.

In the 1920s, the Espee took the next step and converted these compounds into simple-expansion locomotives; see Locobase 6688.

Principal Dimensions by Steve Llanso of Sweat House Media
Locobase ID8728 11,060
RailroadGalveston, Harrisburg & San Antonio (SP)Southern Pacific (SP)
Number in Class1212
Road Numbers950-9614200-4211/3900-3911
Number Built1212
Valve GearWalschaertWalschaert
Locomotive Length and Weight
Driver Wheelbase (ft / m)10 / 3.0511 / 3.35
Engine Wheelbase (ft / m)44.83 / 13.6651.33 / 15.65
Ratio of driving wheelbase to overall engine wheebase 0.22 0.21
Overall Wheelbase (engine & tender) (ft / m)71.75 / 21.8785.08 / 25.93
Axle Loading (Maximum Weight per Axle) (lbs / kg)45,000 / 20,41251,000
Weight on Drivers (lbs / kg)266,000 / 120,656320,100 / 145,195
Engine Weight (lbs / kg)310,000 / 140,614384,800 / 174,543
Tender Loaded Weight (lbs / kg)170,000 / 77,111184,000 / 83,461
Total Engine and Tender Weight (lbs / kg)480,000 / 217,725568,800 / 258,004
Tender Water Capacity (gals / ML)9000 / 34.0910,000 / 37.88
Tender Fuel Capacity (oil/coal) (gals/tons / ML/MT)2850 / 10.803200 / 12.10
Minimum weight of rail (calculated) (lb/yd / kg/m)74 / 3789 / 44.50
Geometry Relating to Tractive Effort
Driver Diameter (in / mm)57 / 144863 / 1600
Boiler Pressure (psi / kPa)200 / 13.80200 / 13.80
High Pressure Cylinders (dia x stroke) (in / mm)21.5" x 30" / 546x76225" x 28" / 635x711
Low Pressure Cylinders (dia x stroke) (in / mm)33" x 30" / 838x76238" x 28" / 965x711
Tractive Effort (lbs / kg)58,070 / 26340.1465,915 / 29898.58
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.58 4.86
Heating Ability
Firebox Area (sq ft / m2)198 / 18.39235 / 21.83
Grate Area (sq ft / m2)53.20 / 4.9470 / 6.50
Evaporative Heating Surface (sq ft / m2)3907 / 362.975292 / 491.64
Superheating Surface (sq ft / m2)512 / 47.57
Combined Heating Surface (sq ft / m2)4419 / 410.545292 / 491.64
Evaporative Heating Surface/Cylinder Volume309.93332.66
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation10,64014,000
Same as above plus superheater percentage11,91714,000
Same as above but substitute firebox area for grate area44,35247,000
Power L160813695
Power MT302.40152.69

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Wes Barris