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Northern Pacific class Z-3
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United States | 1913
21 produced
Image of locomotive
ALCO / collection Taylor Rush

In 1913, the Northern Pacific had its first superheated Mallets made for the operation of heavy, slow freight trains over the Cascades and in the Rocky Mountains. The ten locomotives came from ALCO-Schenectady and had the wheel arrangement 2-8-8-2. Four were coal-fired and destined for the Rocky Mountains, while the other six were oil-fired for the Seattle Division. A total of eleven more from ALCO Brooks followed in 1917 and 1920.

With cylinders measuring 26 and 40 inches in diameter, respectively, they developed more than 85,000 pounds of pulling power. However, these had such a high steam consumption that the highest power was already available at 10.6 mph. In operation, it had been shown that they could haul almost 30 percent more load as pilot compared to a class Z-1 locomotive while consuming the same amount of coal. In the cascades they pulled up to 2,400 short tons alone over most of the run, but were helped by another Mallet on the steeper sections with up to 2.2 percent.

Variant1913 variant1917 variant
General
Built19131917, 1920
ManufacturerALCO
Wheel arr.2-8-8-2 (Mikado Mallet) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Wheelbase55 ft 2 in
Rigid wheelbase15 ft
Total wheelbase83 ft 6 1/4 in
Service weight456,000 lbs483,000 lbs
Adhesive weight399,500 lbs419,500 lbs
Total weight649,200 lbs688,600 lbs
Axle load53,700 lbs56,900 lbs
Water capacity10,000 us gal
Fuel capacity3,500 us gal (oil)32,000 lbs (coal)
Boiler
Grate area84.2 sq ft84.3 sq ft
Firebox area368 sq ft373.6 sq ft
Tube heating area5,129 sq ft5,123.4 sq ft
Evaporative heating area5,497 sq ft
Superheater area1,225 sq ft1,305 sq ft
Total heating area6,722 sq ft6,802 sq ft
Variant1913 variant1917 variant
Power Plant
Driver diameter57 in
Boiler pressure200 psi
Expansion typecompound
Cylindersfour, HP: 26 x 30 in
and LP: 40 x 30 in
Power
Power sourcesteam
Estimated power2,700 hp (2,013 kW)2,800 hp (2,088 kW)
Starting effort85,039 lbf
with start valve102,047 lbf
Calculated Values
steam locomotive
freight
Mallet
last changed: 03/2023
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