Missouri Pacific 4-4-2 "Atlantic" Locomotives of the USA

Class Details by Steve Llanso of Sweat House Media

Class A-79 (Locobase 7093)

Data from 1924 MP locomotive diagram book supplied in May 2005 by Allen Stanley from his extensive Raildata collection. See also builder's card at the SMU digitalRaildata collections site, http://digitalcollections.smu.edu/cdm/singleitem/collection/rwy/id/557/rec/296, last accessed 23 October 2013.

The Atlantics that the MP bought were all built to the same basic design and delivered by the same builder. 5501-5510 came in 1904, 5511-5516 in the same year, and the balance - 5516-5540 in 1907. Most were delivered with Stephenson gear, although some of the last batch arrived fitted with Walschaerts gear operating outboard piston valves.

All were superheated in the late 'teens to the same specifications; see Locobase 7094.

Class A-79 - superheated (Locobase 7094)

Data from MP 7 - 1956 locomotive diagram book supplied in May 2005 by Allen Stanley from his extensive Raildata collection. (Many thanks to Chris Hohl for his detailed update and corrections of this class in a 4 April 2015 email. that prompted a thorough overhaul of this entry.)

A little more than a decade after the Atlantics entered service on the Missouri Pacific (Locobase 7093), the railroad decided to superheat virtually its entire Atlantic holding. It was a pretty straightforward substitution of flues for tubes and little else was changed except the valve gear. About half eventually used Walschaert gear, the other half Baker.

By 1919, only four of the 40 remained to be converted and these were completed by 1924.

A 1926 accounting shows that 38 of the engines burned coal, 2 had been converted to oil-burning. By 1929, twelve were converted to oil burning. Their tenders carried 8,000 US gallons (30,280 litres) of water and 4,250 gallons (16,087 litres) of oil fuel and weighed 148,000 lb (67,132 kg) loaded.

All of these engines and one of the coal burners were fitted with boosters that generated 9,700 lb (4,400 kg) of starting tractive effort. (Two--5513 and 5538-- later rolled on 73" drivers and used 11,020 lb (4,999 kg) boosters. The boosted locomotives weighed 212,000 lb (96,162 kg) in service.

Sixteen 4-4-2s remained on the roster in 1947, but seven were discarded in the next year, and all were gone by 1951.

Still later, the MP added 52 sq ft (4.85 sq m) of thermic syphons to the firebox heating surface area.

Specifications by Steve Llanso of Sweat House Media
ClassA-79A-79 - superheated
Locobase ID7093 7094
RailroadMissouri Pacific (MP)Missouri Pacific (MP)
Number in Class1040
Road Numbers5501-55405501-5540
Number Built10
Valve GearStephensonBaker or Walschaert
Locomotive Length and Weight
Driver Wheelbase (ft) 7.50 7.50
Engine Wheelbase (ft)29.6728.25
Ratio of driving wheelbase to overall engine wheebase 0.25 0.27
Overall Wheelbase (engine & tender) (ft)55.7159.71
Axle Loading (Maximum Weight per Axle) (lbs)
Weight on Drivers (lbs)117,000119,800
Engine Weight (lbs)194,000198,600
Tender Loaded Weight (lbs)124,000124,000
Total Engine and Tender Weight (lbs)318,000322,600
Tender Water Capacity (gals)90009000
Tender Fuel Capacity (oil/coal) (gals/tons)1414
Minimum weight of rail (calculated) (lb/yd)98100
Geometry Relating to Tractive Effort
Driver Diameter (in)7979
Boiler Pressure (psi)200200
High Pressure Cylinders (dia x stroke) (in)21" x 26"21" x 26"
Tractive Effort (lbs)24,67424,674
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.74 4.86
Heating Ability
Firebox Area (sq ft)187238
Grate Area (sq ft)44.8044.90
Evaporative Heating Surface (sq ft)31992548
Superheating Surface (sq ft)504
Combined Heating Surface (sq ft)31993052
Evaporative Heating Surface/Cylinder Volume306.92244.46
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation89608980
Same as above plus superheater percentage896010,507
Same as above but substitute firebox area for grate area37,40055,692
Power L110,44520,904
Power MT393.63769.37


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