Class W - superheated (Locobase 12933)

Data from NP 1 - 1929 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive collection.

Locobases 5357 and 30 show the saturated-boiler originals of the first and largest class of Mikados delivered to the NP. The class was soon superheated using a layout identical to that chosen for the slightly later W-2s, which had larger cylinders.

Class W-1 (Locobase 844)

Follow-ons to the Ws, but fitted with outside Walschaerts valve gear. Initially, they entered service with the boiler pressure set at 150 psi; by 1918, the figure was 180 psi and in 1927, presumably after a reboilering with thicker steel, 200 psi as shown in the specs.

Data confirmed by locomotive diagrams from 1900 hosted on http://www.nprr.org/Steam%20Diagrams/Forms/AllItems.aspx (7 Feb 2004). The diagram shows that the last 5 had Emerson superheaters with one more element and one more flue. Those had a superheater surface area of 570 sq ft.

Class W-2 (Locobase 843)

Data from NP 1 - 1929 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive collection.

Delivered as compounds in 1905, rebuilt and renumbered in 1912 as shown.

Class W-3 (Locobase 2519)

Data from "Heavy Power for the Northern Pacific," Railway Age Gazette, Volume 55, No. 9 (29 August 1913), pp. 377-378, confirmed by locomotive diagrams from 1900 hosted on http://www.nprr.org/Steam%20Diagrams/Forms/AllItems.aspx (7 Feb 2004). By the time this order was completed, according to RAG, the NP had "...in operation 470 locomotives of the 2-8-2 type built by the American Locomotive Company." See also "Extensive Order for New Locomotives for the Northern Pacific Railway Company," Railway and Locomotive Engineering, Vol XXXV, No. 2 (February 1922), pp. 35-36.

These enlarged versions of the basic W class showed the effects of superheating on the layout of tubes and flue. 4 arch tubes of 3 1/2" diameter supported the firebox's brick arch. They also had outside-frame trailing trucks as well as extended piston rods (ahead of the front of the cylinder) to decrease wear on the bottoms of the pistons and hence the cylinders. Piston valves measured a healthy 16" in diameter. Compared to the Q-5 Pacifics built by Brooks in 1920, these Mikes had slightly larger boilers, but less firebox heating surface.

The diagrams show slight differences among several batches of this large class. RAG reports that the first batch did not have combustion chambers, an omission quickly corrected by Superintendent of Motive Power David Van Alstyne. Still later, W-3s arrived with superheaters already fitted in the boilers. The specifications above represent the W-3 as described in the August 1913 article and reflect the inclusion of both the combustion chamber and the superheater.

Ten of this class later went to the Spokane, Portland & Seattle.

Class W-4 (Locobase 846)

Data confirmed by locomotive diagrams from 1900 hosted on http://www.nprr.org/Steam%20Diagrams/Forms/AllItems.aspx (7 Feb 2004).

Six rebuilt from T-class 2-6-2s (Locobase 917) in 1918-1919.

NB: The direct heating surface (including the firebox heating surface) is an estimate calculated by subtracting the calculated tube heating surface from the reported total evaporative heating surface.

Class W-5 (Locobase 847)

Data confirmed by locomotive diagrams from 1900 hosted on http://www.nprr.org/Steam%20Diagrams/Forms/AllItems.aspx (7 Feb 2004) and NP to 1944 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive collection.

Twenty-five engines of considerable power for a Mikado. Most fitted with Elesco or Worthington feedwater heaters. Big 16" (406 mm) valves with 7" (178 mm) travel admitted steam to the nearly square cylinders. The last four were delivered with 55 sq ft (5.1 sq m) of Nicholson thermic syphons.

Drury (1993) cites one 1926 trip in which road number 1844 traveled the 1,898 miles from Seattle to Minneapolis without change pulling a train of unstated weight. During the trip, the locomotive burned 353 tons of coal, boiled 442,000 US gallons (1,672,970 litres) of water and produced 38 tons of ash.

NB: The direct heating surface (including the firebox heating surface) is an estimate calculated by subtracting the calculated tube heating surface from the reported total evaporative heating surface.

Class W/ WA (Locobase 5357)

Data from "Mikado (2-8-2) Locomotive for the Northern Pacific," Railroad Gazette, Volume XXXVIII, Number 3 (20 January 1905), pp. 50-51. See also table in June 1907 AERJ.

First quantity buy of Mikados by any railroad, the 160 ordered by NP had very much a turn-of-the-century look with inside valves and valve gear. Some were built as tandem compounds (in which the 19" high- and 30" low-pressure cylinders were laid out one in front of the other in the same casting.) These first 55 had no combustion chamber and thus had more heating surface area; the firebox heating surface included a mere 9 sq ft in arch tubes. These first locomotives didn't have combustion chambers, he notes, but work with the NP's 2-6-2 demonstrated the value of such an addition and later engines had them.

The RG report describes how this class represented a "...return to rational boiler design, more attention having been paid to circulation and effective heating surface than to an effort to obtain the maximum possible heating surface regardless of efficiency." The writer compares this boiler to contemporaneous Santa Fe Mikados and observes how many fewer tubes the NP engine disposes, for example. Also, handling the lightweight lignite fuel used in NP locomotive required a softer draft than most and the engines were delivered with a squat diamond stack and low nozzle in the smokebox.

RG also flagged the suspension for special attention: "A somewhat unusual arrangement of equalizers has been introduced which divides the total weight into two separate units of two drivers and a truck wheel on each side." The preferred "three-legged stool" of the front and rear driver groups and their associated truck came through a coil spring and transverse equalizer bar for the 2-wheel trucks at each end

Locobase recommends that those readers interested in the details consult the article, but will note that the pattern of suspension was a spring over each of the axle boxes, each pair of wheels on a side coupled through an underslung equalizer between each two suspended axles.

See Locobase 30 for short-boiler variant.

Class W/ WB (Locobase 30)

Data from table in June 1907 AERJ.

First quantity buy of Mikados by any railroad, the 160 ordered by NP had very much a turn-of-the-century look with inside valves and valve gear. The first 55 had long boiler tubes and no combustion chamber; those are described in Locobase 5357. Beginning with 1555, the boiler included a 36"-long combustion chamber that led to the shorter boiler tubes shown in the specs.

Data confirmed by locomotive diagrams from 1900 hosted on http://www.nprr.org/Steam%20Diagrams/Forms/AllItems.aspx (7 Feb 2004). Alfred W Bruce (The Steam Locomotive in America - Its development in the twentieth century (New York: W W Norton, 1952), pp 296-297) says that these Mikes had about the largest grates that could be hand-fired continuously.