Class Z-2 (Locobase 848)
Data confirmed by locomotive diagrams from 1900 hosted on http://www.nprr.org/Steam%20Diagrams/Forms/AllItems.aspx (7 Feb 2004), NP Miscellaneous Steam Loco Diagrams supplied in May 2005 by Allen Stanley from his extensive collection, and Baldwin Locomotive Works Specification for Engines as digitized by the DeGolyer Library of Southern Methodist University, Volume 33, pp. 346-349.
An oddball 1910 design that was fitted with 655 sq ft of "reheater" placed in the smoke box and in the path of the steam exiting the HP cylinders. The idea was to re-energize the exhausted steam before conducting it to the LP cylinders. The boiler also had a feedwater heater with 401 2 1/4" tubes measuring 5' 3" long that was credited with 1,270 sq ft. Because none of this additional area actually superheated anything, Locobase includes it all in the evaporative heating surface total.
Rebuilt 1920-1921 and 1925.
Class Z-2 - superheated (Locobase 10274)
Data confirmed by locomotive diagrams from 1900 hosted on http://www.nprr.org/Steam%20Diagrams/Forms/AllItems.aspx (7 Feb 2004) and NP Miscellaneous Steam Loco Diagrams supplied in May 2005 by Allen Stanley from his extensive collection.
Locobase 848 describes the saturated-boiler version that was delivered in 1910. When the design was superheated, the result was a relatively small articulated locomotive. The later Z articulateds would be considerably larger.
The small tender indicates the role of pusher locomotive on the Bozeman Pass, in which role it performed in 1945.
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 Z-3 - 1913 (Locobase 13133)
Data from "Heavy Power for the Northern Pacific," Railway Age Gazette, Volume 55, No. 9 (29 August 1913), pp. 377-378, confirmed and supplemented by NP TO 1944 Locomotive Diagram book supplied in May 2005 by Allen Stanley from his extensive collection.
Schenectady built the first 10 of these big articulateds in 1913 as the first superheated articulated locomotives on the NP. Brooks added 11 more in 1917 and 1920; see Locobase 849.
Arch tubes contributed 41.6 sq ft of firebox heating surface. It's not clear if the two coal burners (4005 and 4008) shown in the NP diagram book were the last two to use coal or if the split between coal and oil burners had always been 2 and 9. Drury (1993) notes in a caption that the 40"-diameter cylinders, which look like trash barrels mounted horizontally on the front of the engine, "...signify great tractive effort and little speed." And in fact the RAG noted that the maximum horsepower was generated at 10.6 mph. 14" piston valves fed the HP cylinders while slide valves sufficed for the LP cans.
They were widely used throughout the railroad. The RA report described Z-3 service as follows: "Between Helena and Blossburg a Class W-3 locomotive with a Class Z-3 helper is hauling 1,750 tons with no increase in coal over that formerly used by a Class W and a Class Z-1 handling 1,350 tons. This is an increase of 29.6 per cent in train load, with no increase in coal consumption.
"The new Class Z-3 Mallets are used on the Rocky Mountain and Seattle Divisions, four coal burners on the Rocky Mountain Division and six oil burners on the Seattle Division. The grades and trains on the Rocky Mountain Division are described above, these locomotives being used as helpers between Helena and Blossburg.
"The Seattle Division, from Ellensburg, Wash., to Auburn, Wash., is 105 miles long, crossing the Cascade Mountains through Stampede Tunnel west of Ellensburg at an elevation of 2,837 feet above sea level. These locomotives are handling 2,200 tons from Auburn to Ellensburg at a speed of from eight to 14 miles per hour, on a total oil consumption of 2,645 gallons. From Auburn to Lester, a distance of 43 miles, the average grade is one per cent. From Lester to Easton, a distance of 24 miles, there is a ruling grade of 2.2 per cent for ten miles. [A] Class Z Mallet helper ... is used from Lester to Martin. Westbound, the Z-3 locomotive handles a time freight of 1,900 tons from Ellensburg to Auburn on 1,726 gallons of oil, having a Class Z (2-6-6-2) Mallet helper on the 2.2 per cent grade."
Class Z-3 - 1917 (Locobase 849)
Data from "New Locomotives for the Northern Pacific," Railway Age Gazette, Volume 71, No. 17 (29 October 1921), pp. 767-769, confirmed and supplemented by NP TO 1944 Locomotive Diagram book supplied in May 2005 by Allen Stanley from his extensive collection.
See Locobase 13133 for the first 10 of this class delivered by Schenectady in 1913 and a description of their widespread use. The first 5 of the Brooks class arrived in 1917 and weighed slightly less than the 6 that would follow in 1920. The specifications above are taken from the detailed RAG article of 1921.
Arch tubes contributed 41.6 sq ft of firebox heating surface in a boiler that had a slight rearrangement of tube and flue counts. Eight small tubes were deleted in favor of two flues which resulted in an 80 sq ft increase in superheater area. Like the 1913 engines, these articulateds served their HP cylinders with 14" piston valves and their LP trash cans with slide valves.
In 1921, RAG reported that the new Z-3s were assigned to helper service on steep Rocky Mountain and Montana Division grades. On the Seattle division, they hauled the trains themselves, "...handling 2,400 tons between Auburn and Lester, the grade being uniformly 1.0 per cent. Helpers [were] used from Lester to the summit of the Cascade Mountains."
Class Z-4 (Locobase 850)
Data from 1929 NP locomotive diagram book supplied in May 2005 by Allen Stanley from his extensive collection. Works numbers were 64373-64376 in 1923.
Identical to the USRA's heavy articulated design.
None of the NP diagrams showed the firebox heating surface separately, so Locobase took the value from the USRA entry (Locobase 330).