Missouri Pacific 4-8-2 "Mountain" Locomotives in the USA

The Missouri Pacific took delivery of seven 4-8-2s from the American Locomotive Company in 1913. These locomotives, designated Class MT-63, were relatively light (296,000 lbs), and were assigned road numbers 5201 through 5207. They had 63" diameter drivers and exerted 51,075 lbs of tractive effort. The seven "Mountains" were put to work on the St. Louis, Iron Mountain & Southern for the St. Louis - Little Rock passenger traffic.

In 1919, the USRA supplied seven more "Light Mountains" with 69" drivers. They were buit by ALCO and were designated,class MT-69 and assigned road numbers 5301 through 5307.

Between 1921 and 1930 the MoPac took delivery of nineteen class MT-73 "Mountains" from ALCO. These locomotives arrived in four batches: road numbers 5308 through 5312 came in 1921, road numbers 5313 through 5316 in 1923, road numbers 5335 through 5339 in 1927 and the final batch, road numbers 5340 through 5344 arrived in 1930. The last five class MT-73s had 27 x 30 cylinders, 73" drivers, a 250 psi boiler pressure and exerted 63,662 lbs of tractive effort.

In 1939, the Sedalia Shops rebuilt the seven Class MT-69 locomotives that were delivered in 1919. These rebuilt locomotives were given 75" drivers, new boilers, lightweight rods, roller bearings and new tenders. With the new boilers the steam pressure was raised to 250 psi. They were also converted to burn oil and were assigned new road numbers (5321 through 5327) and designated Class MT-75.

There are no surviving Missouri Pacific "Mountains".


ClassRoad NumbersYear BuiltBuilder
MT-635201 - 52071913ALCO
MT-695301 -53071919ALCO
MT-735308 - 53121921ALCO
MT-735313 - 53161923ALCO
MT-735335 - 53391927ALCO
MT-735340 - 53441930ALCO
Note: The seven Class MT-69 locomotives, road numbers 5301 - 5307, were rebuilt in 1939 and were designated Class MT-75 and given new road numbers 5321 through 5327.

Class Details by Steve Llanso of Sweat House Media

Class MT-63 (Locobase 10049)

Data from 1924 MP locomotive diagram book supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. See also "4-8-2 Type for Missouri Pacific", Railway Age Gazette, Mechanical Edition, Volume , 87, No 11 (November 1913), pp. 383-385. (Thanks to Chris Hohl for celebrating his May birthday by emailing Locobase the original tender fuel capacity and loaded weight as well as the firebox heating surface area.) Works numbers were 53715-53721 in July 1913.

There were not at all like the definitive Mountains that would enter service on many roads just a few years later. Indeed, they were more like Mikados with a leading bogie than a true 4-8-2 and indeed their driver diameter reflected freight-like speeds. But they were designed to haul 12-14 car passenger trains. The sticking point on the 165-mile (266 km) St Louis to Poplar Bluff run was a 1.9% ruling grade as well as several sections of 1 and 1.5%, over which a single MT-73 could pull the trainload of 820 tons (50-75% more than earlier locomotives) at 18 mph (29 kph) in thundering majesty.

The RAGME report explained that many of the dimensions were constrained by an adhesion weight limitation of 210,000 lb (92,255 kg). This led to a smaller boiler than one might expect and a smaller grate, which in turn limited cylinder dimensions to squared-up 28" x 28". A short combustion chamber contributed 51 sq ft (4.74 sq m) to the firebox heating surface area. 14" (356 mm) piston valves served superheated steam to the cylinders.

Chris Hohl noted that the class was delivered as coal burners and carried 14 tons of fuel. Loaded weight came to 161,500 lb (73,255 kg), a figure that changed very little when oil fuel was substituted later.

The class clearly provided a useful service because the firebox would later be enhanced with 63 sq ft (5.85 sq m) of thermic syphons, which increased firebox heating surface area to 315 sq ft (29.26 sq m). Three of the seven traded cylinder volume for boiler pressure by having their diameters reduced to 26" and boiler pressure raised to 200 psi (13.8 bar). Starting tractive effort in this trio rose slightly to 51,075 lb (227.56 kN).

They would remain in service until 1948.

Class MT-73 - 1921 (Locobase 14956)

Data from "New Locomotives for the Missouri Pacific", Railway Mechanical Engineer, Vol 95, No 10 (October 1921), pp. 609-613; and F G Lister, "Locomotive Boiler Efficiencey by Proper Circulation of Water", The Boiler Maker, Vol 22, No 9 (September 1922), pp. 256-257. (My thanks to Chris Hohl for his 21 February 2013 inquiry that led to several new entries and updates of several more.) Built by Schenectady in 1921 (5308-5312) and 1923 (5313-5316).

According to the RME report, the MoPac faced contradictory requirements for its heavy passenger engines not long after World War One. Demands for greater capacity and speed meant the MT-63s built in the 'teens simply weren't big enough. At the same time, bridges and other structures forced the railroad to hold adhesive weight in any new design to 113 short tons.

Believing that more efficient use of boiler surface allowed for a smaller boiler, Charles Harter, an MP mechanical engineer devised curved circulating plates, which were estimated to add 10% to the boiler's capacity. At a point just behind the feedwater inlet and slightly below the boiler centerline, the horizontal plate spanned the width of the boiler and extended back to within 4-5" (102-127 mm) of the back tube sheet and 30" (762 mm) of the front sheet. Two-inch (50.8-mm) diameter steam risers were inserted at intervals in the sheet so that they stood vertically among the upper three ranks of 5 1/2" flues and allowed steam generated below the plate to join steam in the upper boiler.

RME reported that the boilers steamed "just as freely as the older ones, are running with 3/4" larger exhaust nozzles, are making better fuel records, and take the same train 30 miles (48.3 km) further for water." Lister quoted Harter as claiming that the boiler raised steam to the desired pressure in 30-45 minutes less time.

In addition to the circulating plates, these engines benefited from a high degree of superheat, 27 sq ft (2.5 sq m) of arch tubes, and 14" (356 mm) piston valves

In later years, the MP rebuilt at least some of the class with new boilers containing 199 tubes and a revamped firebox that had two arch tubes and two thermic syphons. The two additions contributed 13 and 62 sq ft (1.2 and 5.75 sq m) respectively to a total direct heating surface area of 312 sq ft (29 sq m). The design now included an Elesco feed water heater.

See Locobase 211 for the larger and more conventional MT-73s supplied later in the decade.

Class MT-73 - 1927 (Locobase 211)

Data from tables in 1930 Locomotive Cyclopedia. See also MP 7 - 1956 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. (Thanks to Chris Hohl for spotting the error in the valve gear field and for his 21 February 2013 inquiry that led to several new entries and updates of several more.) Built by Brooks (5335-5339) and Schenectady (5340-5344)

Apparently, the MT-73s produced by Schenectady in 1921 and 1923 (Locobase 14956) were still too small for the ever-burgeoning heavy passenger traffic. So they ordered much larger locomotives from Brooks and Schenectady and operated them on routes that featured a beefed-up right of way.

In their final form (shown here), these engines had a generous amount of both superheat and firebox heating surface area. The latter included 18 sq ft (1.7 sq m) in arch tubes and 76 sq ft (7.05 sq m) of thermic syphons. (5337's firebox used Martin circulators totalling 59 sq ft/ 5.5 sq m.)

All of these engines had power reverse gear, BK stokers, Worthington 4 1/4 BL 2 feed water heaters, cast steel locomotive beds, and roller bearings.

Principal Dimensions by Steve Llanso of Sweat House Media
ClassMT-63MT-73 - 1921MT-73 - 1927
Locobase ID10,049 14,956 211
RailroadMissouri Pacific (MP)Missouri Pacific (MP)Missouri Pacific (MP)
Number in Class7910
Road Numbers5201-52075308-53165335-5344
Number Built7910
Valve GearBakerWalschaertWalschaert
Locomotive Length and Weight
Driver Wheelbase (ft / m)16.50 / 5.0319.58 / 5.9719.58 / 5.97
Engine Wheelbase (ft / m)36.83 / 11.2341.33 / 12.6042.17 / 12.85
Ratio of driving wheelbase to overall engine wheebase 0.45 0.47 0.46
Overall Wheelbase (engine & tender) (ft / m)70.52 / 21.4977.17 / 23.5288.37 / 26.94
Axle Loading (Maximum Weight per Axle) (lbs / kg)
Weight on Drivers (lbs / kg)208,000 / 94,347226,000 / 102,512267,500 / 121,336
Engine Weight (lbs / kg)296,000 / 134,264335,000 / 151,954396,000 / 179,623
Tender Loaded Weight (lbs / kg)163,000 / 73,936192,800 / 87,453320,000 / 145,150
Total Engine and Tender Weight (lbs / kg)459,000 / 208,200527,800 / 239,407716,000 / 324,773
Tender Water Capacity (gals / ML)8000 / 30.3010,000 / 37.8814,000 / 53.03
Tender Fuel Capacity (oil/coal) (gals/tons / ML/MT)4250 / 16.1016 / 14.5018 / 16.40
Minimum weight of rail (calculated) (lb/yd / kg/m)87 / 43.5094 / 47111 / 55.50
Geometry Relating to Tractive Effort
Driver Diameter (in / mm)63 / 160073 / 185473 / 1854
Boiler Pressure (psi / kPa)170 / 11.70210 / 14.50250 / 17.20
High Pressure Cylinders (dia x stroke) (in / mm)28" x 28" / 711x71127" x 30" / 686x76227" x 30" / 686x762
Tractive Effort (lbs / kg)50,350 / 22838.4053,477 / 24256.7963,663 / 28877.09
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.13 4.23 4.20
Heating Ability
Firebox Area (sq ft / m2)285 / 26.48327 / 30.38451 / 41.90
Grate Area (sq ft / m2)56.30 / 5.2367 / 6.2284.30 / 7.83
Evaporative Heating Surface (sq ft / m2)3455 / 320.983934 / 365.485092 / 473.06
Superheating Surface (sq ft / m2)785 / 72.931084 / 100.711352 / 125.60
Combined Heating Surface (sq ft / m2)4240 / 393.915018 / 466.196444 / 598.66
Evaporative Heating Surface/Cylinder Volume173.14197.88256.13
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation957114,07021,075
Same as above plus superheater percentage11,38917,16525,501
Same as above but substitute firebox area for grate area57,65683,777136,428
Power L110,68519,69529,858
Power MT453.01768.50984.31

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