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. 583-585. (Thanks to Chris Hohl for celebrating his May birthday by emailing Locobase the original engine and total wheelbases, 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. On the other hand, total engine and tender wheelbase decreased to 68 ft 1 in (20.75 m).

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 (works numbers were 67311-67315 in July 1927, road numbers 5335-5339) and Schenectady (works numbers were 68338-68342 in June 1930, road numbers 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.

Class MT-75 (Locobase 16451)

Data from MP 1956 Locomotive Diagrams (Steam) supplied in August 2013; and Joe G Collias, Mopac Power: Missouri Pacific Lines locomotives and trains, 1905-1955 (Howell-North Books, 1980), pp. 01-202, 207, & 211-217 . (Many thanks to Chris Hohl for his 21 September 2019 email notifying Locobase of the MT-75 rebuilds and for supplying a very detailed description of the changes and additions that appears below.)

Chris Hohl writes:

These locomotives started their lives as USRA light 4-8-2s [Locobase 231] built by ALCO-Richmond in December 1919,entering service as Missouri Pacific's MT-69 class 5301-5307. Their "as delivered" Sunbeam headlight was quickly replaced with a Pyle headlight, and they all had a Worthington Type 4-S-2 feedwater heater system installed by 1935. The shops also slowly, but steadily, replaced their USRA-standard Alco Cole-Scoville tender trucks with Commonwealth tender trucks. (Collias describes the USRS Alco trucks as "unique top equalized 4 wheel tender trucks so peculiar to passenger power orignated by this authority." Collias reported that MoPac assigned the updated 5301s to "heavier passenger assignments on the Arkansas division between Poplar Bluff, Mo and Texarkana , Tex.

In 1939, the MoPac needed to shorten the time of their Royal Gorge schedule, compete with several different diesel-powered streamliners, and keep their costs down at the same time. So, the Sedalia shops brought the MT-69 class 4-8-2s in and rebuilt them. Essentially, Joe Collias said, they emerged as brand new locomotives with only small portions (which included the back ends of their boilers, their domes, their cabs, and their stacks) retained from their "as-built" state. Collias adds that the result was "most modern steam engine yet on the system and, if the word may be used to describe a steam locomotive, a most elegant design, elegant in the sense of being modernly [sic] so."

Boiler pressure in the new boilers rose by 25 psi (1.72 bar) and cylinder diameters decreased 1/2" (12.7 mm). While tractive effort fell slightly, their speed and horsepower ratings increased quite substantially. Firebox heating surface included 72 sq ft (6.69 sq m) in Nicholson thermic syphons.

To achieve the higher speeds the MoPac needed for their portion of the Scenic Limited, these locomotives had their driver diameters raised to 75" (which made them the highest drivered 4-8-2 Mountain-type ever built in North America. In fact, Locobase has only three 4-8-2s--all French--with taller drivers; see Locobases 1062 (Est), 6071 (PLM), and 1064 (SNCF)), and their drive wheels were changed from spoked to Baldwin "aesthetically tasteful" (Collias) disc drivers. These locomotives were also the first on the Missouri Pacific to have Timken tapered roller bearings used throughout the locomotives and tenders, and their Baker valve gear was replaced with Walschaert. This change required these "rebuilt" locomotives to also receive new cast engine beds.

The locomotives were also converted from coal-burners to oil-burners, and they were given brand new tenders built by the Sedalia shops. Their new tenders originally held 5,650 US gallons (21,385 litres) of oil & 15,000 gallons (56,775 litres) of water, but, later in their careers, they had their capacities changed in multiple different ways throughout the class. One of their most unusual changes (in comparison to the MoPac's other locomotives) was that the new locomotives received a "solidly smooth surface cast steel pilot with vertically retractable coupler". While pilots of this nature weren't completely new by this point, the MoPac's differed with the design being more angular and tapered in shape around the storage area for the coupler than other designs used on other lines (such as the Northern Pacific and the Milwaukee Road).

5321-5323 were finished in October 1939, and 5324-5327 were finished in March 1940. The only appearance changes to the class after their rebuilds were a conventional pilot refitted to 5322 after an accident and a red "Mars" light equipping the 5326.

The new locomotives were allowed a maximum speed of 90 mph (145 kph) --a "commonplace" occurrence according to Collias--in order to meet their new timings hauling trains from St. Louis to Pueblo, CO. Moreover, he noted, the locomotives were so efficiently scheduled and maintained that they averaged 560 miles (902 km) per day. These masterpieces continued to work, almost exclusively, in passenger service until they were eventually put into storage and, finally, scrapped in 1954.

Principal Dimensions by Steve Llanso of Middle Run Media

ClassMT-63MT-73 - 1921MT-73 - 1927MT-75
Locobase ID10049 14956 211 16451
RailroadMissouri Pacific (MP)Missouri Pacific (MP)Missouri Pacific (MP)Missouri Pacific (MP)
Number in Class79107
Road Numbers5201-52075308-53165335-53445321-5327
Number Built79107
BuilderAlco-SchenectadyAlco-SchenectadyAlco - multiple worksMP
Valve GearBakerWalschaertWalschaertWalschaert
Locomotive Length and Weight
Driver Wheelbase (ft / m)16.50 / 5.0319.58 / 5.9719.58 / 5.9719.50 / 5.94
Engine Wheelbase (ft / m)36.42 / 11.1041.33 / 12.6042.17 / 12.8542.33 / 12.90
Ratio of driving wheelbase to overall engine wheelbase 0.45 0.47 0.46 0.46
Overall Wheelbase (engine & tender) (ft / m)70 / 21.3477.17 / 23.5288.37 / 26.9475.70 / 23.07
Axle Loading (Maximum Weight per Axle) (lbs / kg)
Weight on Drivers (lbs / kg)208,000 / 94,347226,000 / 102,512267,500 / 121,336244,380 / 110,849
Engine Weight (lbs / kg)296,000 / 134,264335,000 / 151,954396,000 / 179,623370,100 / 167,875
Tender Loaded Weight (lbs / kg)163,000 / 73,936192,800 / 87,453320,000 / 145,150310,000 / 140,614
Total Engine and Tender Weight (lbs / kg)459,000 / 208,200527,800 / 239,407716,000 / 324,773680,100 / 308,489
Tender Water Capacity (gals / ML)8000 / 30.3010,000 / 37.8814,000 / 53.0315,000 / 56.82
Tender Fuel Capacity (oil/coal) (gals/tons / Liters/MT)4250 / 16,08616 / 1518 / 165650 / 21,385
Minimum weight of rail (calculated) (lb/yd / kg/m)87 / 43.5094 / 47111 / 55.50102 / 51
Geometry Relating to Tractive Effort
Driver Diameter (in / mm)63 / 160073 / 185473 / 185475 / 1905
Boiler Pressure (psi / kPa)170 / 11.70210 / 14.50250 / 17.20225 / 15.50
High Pressure Cylinders (dia x stroke) (in / mm)28" x 28" / 711x71127" x 30" / 686x76227" x 30" / 686x76226.5" x 30" / 673x762
Tractive Effort (lbs / kg)50,350 / 22838.4053,477 / 24256.7963,663 / 28877.0953,722 / 24367.92
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.13 4.23 4.20 4.55
Heating Ability
Tubes (number - dia) (in / mm)218 - 2" / 51182 - 2.25" / 57239 - 2.25" / 57213 - 2.25" / 57
Flues (number - dia) (in / mm)32 - 5.375" / 13740 - 5.5" / 14050 - 5.5" / 14040 - 5.5" / 140
Flue/Tube length (ft / m)20 / 6.1022 / 6.7122 / 6.7122 / 6.71
Firebox Area (sq ft / m2)285 / 26.48327 / 30.38451 / 41.90396 / 36.79
Grate Area (sq ft / m2)56.30 / 5.2367 / 6.2284.30 / 7.8370.80 / 6.58
Evaporative Heating Surface (sq ft / m2)3455 / 321.103934 / 365.485092 / 473.064404 / 409.14
Superheating Surface (sq ft / m2)785 / 72.961084 / 100.711352 / 125.601261 / 117.15
Combined Heating Surface (sq ft / m2)4240 / 394.065018 / 466.196444 / 598.665665 / 526.29
Evaporative Heating Surface/Cylinder Volume173.18197.89256.14229.97
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation957114,07021,07515,930
Same as above plus superheater percentage11,38917,16525,50119,435
Same as above but substitute firebox area for grate area57,65683,777136,428108,702
Power L110,68819,69629,85826,044
Power MT453.13768.53984.31939.80

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