Northern Pacific 2-6-6-2 "Mallet Mogul" Locomotives in the USA

Class Z (Locobase 1046)

Data confirmed by locomotive diagrams from 1900 hosted on [] (7 Feb 2004) and Tom Farin's [] . See also Baldwin Locomotive Works Specification for Engines as digitized by the DeGolyer Library of Southern Methodist University, Volume 30, pp. 16-17. (Thanks to Chris Hohl for his 31 December 2016 email pointing out that the class was delivered as coal burners.) Works numbers were 30566 in April 1907; 31185-31187, 31245, 31307-31308, 31389-31394 in July; 31484, 31514, 31557 in August.

Identical to the Great Northern's L-1s (Locobase 819) and fitted with a Belpaire firebox. Placed in helper service where they were rated at 850 tons up 2.2% grades. These were slow-speed engines, held by the railroad to 15 mph (24 kph) or less.

Among the details found in Baldwin's specs is this one concerning the valve motion: "Make the exccentric diameter on the LP [low-pressure] cylinder 18" [457 mm] and 15" [381 mm] diameter on HP [high-pressure] making eccentric rod connection on link to correspond." The ability to tweak the Walschaert gear design as closely as this was one of its greatest appeals to operators.

A 10 December 1907 report in the Baldwin specs file summarized interviews held with General Manager George P Slade, Superintendent of Motive Power William Moir, and Mechanical Manager W L Kinsell. "The above gentlemen, although interviewed separately, united in commenting favorably on the work done by the Mallet compound locomotives now operating west of Livingston."

The letter goes on to report on the "howevers" that such an opening often preceded. Most were relatively minor, as in the ash pans being too level and too close to the grate, which led to grate-bar burnthroughs. A criticism of the starting-valve pipes' small diameter, which the railroad had remedied by installing 1 1/4" (31.8 mm) pipes, was annotated "These were specified to be 1" [25.4 mm] dia."

But "the most serious trouble complained of is due to the loosening and leaking of the studs holding the bearing saddle to the waist of the boiler." Not only was repair time-consuming and immobilizing, but the steam and water leakage onto the bearing surfaces might lead to derailments, they feared.

Chris Hohl reminded Locobase that these locomotives were delivered as coal burners trailing tenders holding 13 tons (11.79 metric tons) of coal. 3005, 3008, and 3013-3015 were modified to burn oil in February 1913, as a result its tender carried an unusually precise 3,449 US gallons (11,793 litres) of oil fuel. 3003-3004 and 3006 received their burners in October. Nine years later, 3000-3001 were fitted with oil burners in August 1922 and 3002 in March 1923

Meanwhile, 3003 gave up its burner in November 1918 and was converted back to coal firing. Apparently the railroad decided to convert the other oil burners to coal firing in 1925 -- May (3000-3001, 3004-3006) and July (3014-3015).

Twelve of the class were retired in 1926-1927. 3015 lasted until February 1930. Three survived until 1939 when 3014 left in March and 3001 and 3005 retired in May.

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Class Z-1 (Locobase 821)

Data from DeGolyer, Volume 33, pp. 350+. See also locomotive diagrams from 1900 hosted on [] (7 Feb 2004). (Thanks to the Locobase visitor whose 28 December 2018 email pointed out the unusual, for the time, provision of piston valves on all four cylinders.) Works numbers were 34389-34390 in March 1910; 34422, 34451-34452, 34483 in April.

Follow-ons to the Zs (record 1046), but significantly lighter and fitted with smaller cylinders. As with the Zs, these were a copy of a Great Northern design, in this case the L2 (Locobase 3554). They had straight boilers, 13" (330 mm) piston valves serving steam to both the HP and LP cylinders, and small fireboxes with wide grates.

3103 was retired in August 1932, 3101 in June 1934, and 3104-3105 in October 1934. 3102 lasted until October 1940

3100 was sold to Polson Logging Company in December 1940. Operating out of Railroad Camp, Wash, the 3100 operated until 1945 with the Polson herald on its tender, then for successor company Rayonier, Inc until 1959.