Quoting a Northern Pacific "Tell Tale" article from its February 1950 issue, the site says that these two decapods were intended for heavy freight service in the Cascade mountains and were the heaviest locomotives in the world at the time they entered service.
Engineering reported that the pair had been "a surprise to the mechanical department of the system, owing to the unexpected ease with which they pass round the sharpest curve." This echoed a letter sent by the NP's Superintendent of Motive Power G W Cushing in January 1887 to Philadelphia, who wrote that the Ms could go "anywhere, either on bad track or curves, that an eight-wheeled engine could go." (reported in "Stampede Pass Switchback and Tunnel", Railroad Gazette, Volume 19 (23 December 1887), pp. 824-825.
At the time the NP was building switchbacks to work trains over the Cascades and the Ms would provide helper service. The Stampede tunnel, which would reduce maximum grades to 116 ft/mile (2.2%), was expected by the end of 1888.
The pair were later renumbered 1-2 and converted to yard switchers.
|Specifications by Steve Llanso of Sweat House Media|
|Railroad||Northern Pacific (NP)|
|Number in Class||2|
|Builder||Burnham, Parry, Williams & Co|
|Locomotive Length and Weight|
|Ratio of driving wheelbase to overall engine wheebase||0.68|
|Overall Wheelbase (engine & tender)||49.17'|
|Axle Loading (Maximum Weight per Axle)||27000 lbs|
|Weight on Drivers||133000 lbs|
|Engine Weight||148000 lbs|
|Tender Light Weight||77530 lbs|
|Total Engine and Tender Weight||225530 lbs|
|Tender Water Capacity||3500 gals|
|Tender Fuel Capacity (oil/coal)||7 tons|
|Minimum weight of rail (calculated)||44 lb/yard|
|Geometry Relating to Tractive Effort|
|Boiler Pressure||150 psi|
|Cylinders (dia x stroke)||22" x 26"|
|Tractive Effort||34880 lbs|
|Factor of Adhesion (Weight on Drivers/Tractive Effort)||3.81|
|Firebox Area||162 sq. ft|
|Grate Area||35.70 sq. ft|
|Evaporative Heating Surface||2310 sq. ft|
|Combined Heating Surface||2310 sq. ft|
|Evaporative Heating Surface/Cylinder Volume||201.94|
|Computations Relating to Power Output (More Information)|
|Robert LeMassena's Power Computation||5355|
|Same as above plus superheater percentage||5355|
|Same as above but substitute firebox area for grate area||24300|